Well Karma is exactly a theory that tries to explain the evil in the world. Evil is like the first thing people notice (like the first Noble Truth of Buddhism says, "there is suffering"!), and that's why every religion tries to explain it, in a different way in every case.

The idea of Karma is meant to help people accept their suffering, so they can cope with their shitty lives and with injustice, and also maybe so they won't try to do anything about it. Science solves problems, it says "if you are suffering, let me help you". Karma says "if you are suffering, you deserve it". It's actually a very nasty idea. So imagine a 10-year-old girl gets raped. Now imagine telling her she deserves it! A 'Karma rape counselor' would be a very unpopular person indeed!

Here are a few problems with the idea of Karma: First, let's go back, let's go waaay back. You say I suffer X because I did Y. So let's go back to the first person who ever did an evil thing. Say his name is John, and John did something bad to James. Why did James suffer without any reason? So, there must've been the problem of the first person (or animal) who suffered. Either this, or everyone had past lives to infinity, which is ridiculous. So Karma doesn't answer the question ('the problem of evil'), it just pushes it back.

Second, it means we never know what's good and what's bad. Maybe the girl who got raped was a rapist in her former life. So the rapist who raped her, wasn't doing anything bad. So maybe he won't get punished? How can anyone live their life like that, never knowing what's good and what's bad?

Third, there are many more humans now being born than in any other point in history. Where are all these souls coming from? The animal population mathematics don't exactly add up.

Etc.

Killing should not be called an act of hatred until we are assured of what happens to us after death. Isn't it ?

True. I always wondered why Christians don't celebrate when a person dies! But this is exactly the reason Nietzsche called Christianity and Buddhism nihilist religions: they look favorably upon all things that are anti-life. So for example scientific/material immortality is within our grasp, but we don't pursue it because people believe in immaterial immortality, so they don't care about real immortality (the same way people used to refuse treatments from doctors because they said that God willed this disease for them, so treating it would be against God's will).

In general, before I put my faith in X, I would like to see some evidence for X. I see no evidence for Karma, or life after death, and I see many problems with these ideas. So I'd rather put my faith in science, that actually tries to solve problems instead of rationalizing them.