There are many opinions about the existence of evil and religions usually have two ways of dealing with it:
1st) God has no power over human's free will. In other words, God can't stop a person A from killing a person B because He would interact directly with the free will. In other words, God wants to help, but He can't when free will interferes.
2nd) Afterlife. This world is a test in every religion. Because of that, both good and evil things happen in this world, but in the end the good people are going to go to heaven and evil ones will fall into hell. In other words, evil in this word doesn't really matter, because justice will prevail in the afterlife.
My point of view: I am an Orthodox Christian. However, i don't really care about afterlife, because a true believer loves the whole world as much as himself/herself, so he/she does the right thing because he/she likes it and believes to it. A true believer doesn't think "I have to do this because I will go to heaven, I mustn't do that because I will burn to hell" (in fact, this is self-interest and not true love), he/she rather thinks "I will do that becaue it is the right thing to do, I love people, so I am going to help people". I also don't care about evil deeds of others, because I don't really think I should judge them (since I am imperfect too), but I should persuade them to stop instead. Furthermore, despite of all the evilness in the word, I shouldn't think "everyone is evil, let me become evil too!", but I should try to remain faithful to the righteous way of altruism. That's my point of view and I think it is probably right (however, I have to admit I fail to live through my beliefs).