Why would lack of belief in a deity lead to evil?

The reason is that Goodness resides in God alone. It is not something that exists outside of God.
Alienation from God is alienation from goodness, and love and truth and peace and every other good thing that comes only through God.

THis earth depends entirely upon the sun for its life. Without the sun all life would cease in a matter of days if not in a matter of moments.
In the same way man was never meant to exist apart from God. That is why God calls it being lost.

Jesus said "You must be born again", because sin kills; but God makes alive.....

http://4laws.com/laws/english/flash/

Because they are cognitively challenged.

Because they're stupid.

EVIL=A Complete Violation Of God's Holy Purpose! period.

Because for some people, the only thing that keeps them moral is the threat of eternal damnation by god if they are immoral, so they think anyone who doesn't believe in eternal damnation must be immoral because there's nothing to keep them moral.

because they try to scare you with threats of evil as mind control. There many evil "believers" out there too so dont let that scare you.

Hell if I know, no pun intended

Because some people believe that if you don't believe in God then you have basically rebelled against the path he meant for you. God is all knowing, all loving, all powerful, etc etc, you know the drill, so therefore you're going against the good and joining with evil.

Because they are not protected by God
And they don't know right from wrong according to the bible.

Because they think morality comes from their God.

Know how I know it doesn't? Because if our morals came from one true, eternal God, then different societies would not have different morals, but they do. And our morals would not change over time, but they do (hence, we used to think slavery was moral, but don't anymore)

The fact is, that people who believe in God are the ones who commit the most immoral acts, such as honor killings, discrimination and so forth.

Even here in the States, the places with the highest concentration of Christians have the highest statistical rates of immoral behavior such as sexually transmitted disease, teen pregnancy, spousal abuse, child abuse and criminal recidivism.
http://community-2.webtv.net/tales_of_the_western_world/BIBLEBELT/
http://www.stopthereligiousright.org/biblebelt.htm

There is absolutely no FACTUAL indication that religious belief leads to more moral behavior. Quite the opposite.

Didn't those Bible Belt Christians, the Southern Baptists and Methodists in the 1950's and 1960's, believe in God when they lynched 3,500 innocent black men, women and children? Of course they did...they lynched those people and then went to their Sunday services and their potluck church suppers like everything was fine.
http://en.wikipedia.org/wiki/Lynching

@Eddie: It's the religious that think they can be absolved from personal responsibility by asking forgiveness. Atheists understand we always have personal responsibility for our actions. It is not atheists who attempt to circumvent morality. That's why the rate of crime is so much higher for Christians than for atheists. Statistically, atheists are 14% of the population of the states, but we are less than 1% of the prison population.
http://www.skepticfiles.org/american/prison.htm
Christians make up 76% of the population, but 81% of the prisons.

It doesn't.

People believe it does because most religions tell them to believe in god or go to some hell.

And only "evil" people go to hell. The logic is actually pretty simple (and false, but simple still)

They are using the same scare tactics that work against them and caused them to belief in the imaginary. Secretly, they wish it would scare us into believing too.

Remember: Only the Sith work in absolutes.

As long as you're not openly campaigning you atheism, I would think most religious people would be okay with it. Apparently not. I think the best way to go would be to do good in the world, and if there is a god, surely you've pleased him, if not, at least you made the world a better place. No need to believe in a specific anything. That would be a perfect world.

Because they are delusional beings and therefore dangerous to a safe society and if there were any evil going on that's where i would look .

You mean like Red China? Communist Russia?

Evil is subjective, Karl Rove and Dick Cheney think not murdering babies in Iraq is "evil".
The rest is easy to figure out, you should be able to take it from there.

because anything that goes against their belief system is evil in their point of view. Point out that it is only under the guise of religion that the worst atrocities in human history were committed so given the choice, you have really chosen to join the good.

they could also be making the assumption that because they lack the intelligence / moral fiber to know the difference between good and bad that therefore everyone else is incapable of such and in need of an antiquated book of fables to guide them.

I don't think it does lead to directly lead to evil. What it does do is remove a "safety net" in the sense that a good religion gives us an "external reference" by which we can compare our intentions or actions to ensure they aren't evil. Without this "external reference", there is only our conscience, moral beliefs, and local laws to act as a brake on the slippery slope down towards Hell, the problem with them is that our sinful hearts won't shout "STOP" when we see an injustice. This is also the reason people regularly trot out the "but religion is the cause of wars" type of argument, because people with sinful hearts cherry pick some scriptures to justify sin and didn't adhere to founding ideals (e.g. the 10 Commandments), the cause of the evil being the sinful hearts of the leaders, not the religion.
On the other hand, a belief in a deity and the correct use of scriptures, e.g God and The Bible, will tell us what is right or wrong regardless of whether we believe this or that is right.

They are mislead. some people because they are taught that the devil is evil and the ruler of this world though the devil does rule this world he is a liar and he is not necessarily always evil in the sense we define evil. The Bible teaches that all are sinners and slaves to sin but it does not say all sinners are evil. nor does it say that without God you are evil.

People are just as easily mislead who believe in God as those who do not. There are many lies out there that are partly true and too many people think if it is partly true then it is entirely true. Very much the same reason people believe in evolution they hear it from people who have degrees in studies so they assume the person knows what they are talking about. Others think that because a person calls himself/herself a minister or pastor or priest or whatever they think they know something. Titles and degrees do not mean people know truth or even any knowledge of things it just means they have either claimed the title or gotten a degree. If a person actually earns the degree and yet the teaching they received is based on lies then their knowledge is based on lies.

Just like not believing in a "Good" diet and well-managed eating lifestyle might lead to a heart attack and high cholesterol and kidney failure and cancers later in life. Who knows? Can we prove it? Sort of. Depends on who you talk to. You could just wing it....and eat moderately.... whatever you like whenever you like and hope for the best (atheist). Or you could be on the good path and really watch out for your soul everything you put in your mouth....(Christian).....Or you could eat cheeseburgers everyday..I won't use an example there...lol. But basically none of us REALLY knows what's what. It's all a matter of faith or lack there of. Some of have found a reason to believe in something more...some of us figure there is no reason it just is. Ever since the dawn of time it seems people have looked to the heavens and pushed themselves to a higher potential. If that isn't God at work......who knows. Some will always find a reason to believe in something, and some will find a reason to push it away.
Being an atheist doesn't designate someone as a servant of evil but it does mean they have pushed away the possibilities of "more". The potential of what we are, where we might end up, and why it is all the way it is may not be completely explained accuratelty in any religious dogmatic text but I find so many young atheists use that as an excuse to disjoint themselves from spirituality completely and justify it by saying "but I'm still a GOOD person" right? Well of course. It just depends on what you mean by Good and BAD. Are you living up to your full potential? ARe you shining to your full capacity, who knows?

? yep! but then!!

who do YOU fear?

those who know god goes to heaven those who dont are ghost those who do evil go to hell...simple as that