Why did Jesus go in the temple and beat the money changers with a whip?

He didn't. He turned over their tables and scattered their money all over the floor. They were scamming the people.

They weren't just selling things, they were selling things way overpriced, taking advantage of the fact that people could not buy the things they needed to use in the temple anywhere else.

Your ignorance and incompetence are showing 

He certainly made a whip of cords, using that to gee-up the animals to drive them out, and He overturned the tables of the money-exchangers. Imagine the chaos! Doves flying up and away, cattle lowing, sheep bleating and charging all over the place, the men frantically trying to recover their scattered piles of coins as Jesus shouted in anger at them. But it doesn't say anywhere that Jesus beat the men with a whip. Read John 2:12-17.

As to why He did what He actually did, it was due to liberties being taken within the Court of the Gentiles. Pilgrims coming to the Passover Feast then needed animals that met the ritual requirements for sacrifice, and the vendors set up their animal pens and money tables in that outer court. Pilgrims needed to change their money into local currency as the annual temple tax had to be paid in that currency. But Jesus was no fool as to unfair trading. Another problem that aroused Jesus' ire (as Mark details) was the temple courts being used as a short-cut between the city and the Mount of Olives.

By allowing the Court of the Gentiles to become a noisy, smelly market-place, the Jewish religious leaders showed disregard for that area being a place of prayer and worship for all nations (i.e. Gentile nations). Not only were the Jewish traders taking financial advantage of visitors, they were robbing the temple of its sanctity. The wonder is that Jesus retained self control so that He never struck another person!

Because they were making money for themselves right in God’s house of worship. Jesus told those who were selling doves: ‘Take these things away from here! Do not make my Father’s house a place of business!’ He would not let anyone show disrespect for his Father’s house.

You always make up things?  No whip.  And the reason was that the money-changers were fleecing the faithful.

The reason is found here,please read Before long, Jesus and his disciples reach Jerusalem. He goes to the temple, which he inspected the previous afternoon. Today he does more than make an inspection; he takes action similar to what he did three years earlier at the Passover of 30 C.E. (John 2:14-16) This time Jesus throws out “those selling and buying in the temple.” He also overturns “the tables of the money changers and the benches of those selling doves.” (Mark 11:15) He does not even let anyone carrying things to another part of the city take a shortcut through the temple courtyard.
Why is Jesus taking decisive action against those changing money and selling animals in the temple? He says: “Is it not written, ‘My house will be called a house of prayer for all the nations’? But you have made it a cave of robbers.” (Mark 11:17) His reason for calling these men robbers is that they demand exorbitant prices from those who have to buy animals needed for sacrifice. Jesus views their dealings as extortion, or robbery.

They broke the rules of His house. What would you do if strangers were in your house with prostitutes, putting their cigarettes out on your $2000 rug, and doing illegal drugs? Those are the rules for YOUR house and you'd probably kick them out. 

And, don't be cute by saying I'd welcome the girls and drugs. I've already thought of that.

He knew he'd be crucified. , and wasn't happy 

Because he forgot his baseball bat in his other suit?