i dont understand ? MOSTLY republicans...?

Rightwings are against killing innocent children whom haven't even had the chance to see daylight.

But, because killers and other serial criminals have no concern for human life, and have proven themselves to be subhuman, they deserve death.

Is it that hard to comprehend?

OK. Try this: Republicans are avid defenders of human life, right up to the time the child is born. After that, it is a taker, an immigrant anchor, a reason for a mother to receive welfare, a disgusting consumer of school lunch, head start, Medicaid, and a host of other mandatory programs that prevent the 1% from taking more dollars from the tax base.
Strangely, the Evangelical Christians join in and promote these positions. WWJD?

Conservatism is a very honest ideology.
So it is hopeful, but it is also very unforgiving.

In simple terms, for the purpose of this question, one of the basic tenets of conservatism is "freedom and responsibility."
That word, "responsibility," is very popular right now, since a lot of people are preaching for safety, for new policies to protect ourselves from the negative consequences of bad decisions.
Such policies are often tossed into the big, expensive, government system called "welfare."

Essentially, conservatives don't see freedom as anything that can be guarded by higher powers. Any time there is a proposal that the government interferes in policy, that people can be rescued by policy, then that is, mathematically and financially, the beginning of a chain reaction where, inevitably, one man's misuse of his freedoms eventually causes another man to lose a portion of his freedom in order to reinforce the loser (usually financially, which both costs a person his efforts at work but also empowers a government to demand higher taxes in any case, setting a bad precedent for for future expectations).
And although conservatives are big on charity, hard on crime, and known for attempting to uphold such high moralistic views that they almost can't help but to slip into hypocrisy now and then, the allowance of infringement of their freedoms to protect against those failings is abhorrent.
Why? Because freedom cannot exist alone. It almost can't exist at all, since the de facto human nature is selfish and greedy. Freedom was paid for with blood, but and in order to maintain it, people must see it as an equally serious issue as they did when, for the first time in history, less than 300 years ago, it was conceived. That means that people MUST be responsible to their freedom. We must fight, viciously, to protect it and prevent the everpresent encroachment of the will of the powerful, the weak, the greedy, and the morally repugnant.
And that means No Mercy.

So I could go on a lot longer, but think about what I just wrote for a minute and then rethink the argument against abortion versus the indifference toward the death penalty.

Abortion is the extermination of an innocent life. Murder - the ultimate defiance against freedom. It is done with reasons pertinent to protecting a person from the consequences of his/her own choices, which is essentially submission of reason to possess freedom at all to those who have power, to those who are weak, to those who are selfish.
It is not a conservative value, and it is seen, like many other things, as a potential threat to anybody who fights for the right to live life as his own person rather than clinging to a master or a protector to save us from ourselves (ie. take away our freedom).

Death Penalty: The extermination of a non-innocent life. Is it murder? That's debatable, which is why some conservatives still aren't keen on it, myself included. However, there are many instances where a death penalty is understood to not be a murder. In war, in a car crash, in a suicide, in a machine's malfunction, in a Darwin Award (a little dark humor there), we see people suffer death for the mishandling of their freedom - or sometimes as a consequence of engaging in an activity that, though not intended to cause death, can occasionally be fatally accidental. Unfortunately, these deaths occur. They are not murders, but they do, and with great frequency, occur to cut short the lives of people who have made decisions, as per their freedom, to engage in various activities and then abide the consequences.
Sometimes the consequence is death, which is tragic. Sometimes it's bankruptcy. Sometimes it's the dissolution of a marriage. Sometimes it's a million dollar win at the casino! Whatever happens, we are free to do our best to encourage our will, and then, as per the RESPONSIBILITY that comes with freedom, we endure the effects, be they good or bad.
So the person suffering the death penalty is not a consequence of another person's actions (as is the case with the one who faces death by abortion); instead, the penalty he faces is a known and understood variable consequence of actions that he had the freedom to engage in and the responsibility to endure as a free person. Anything else, in this case, would be an affront to the concept of freedom, just like murder itself (which is probably the thing he did to earn the death penalty), just like any other situation where a life is robbed unjustly or without cause.

So... I guess that's it.

How does it not? Two totally unrelated things.