1) What Islam stances about killing? I have heard from someone, it stated that in our literature it is forbidden to kill a Muslim, but it does not mention it is forbidden to kill a non Muslim.

2) I want to ask that, is it true the punishment for someone who left Islam or those who ommited the five prayers intentionally is death penalty?

3) If someone oppressed us for example spread lies about us, slander about us, and etc. can we retaliate through killing? And when does Islam permits killing of another human being

1) It is forbidden to kill anyone, Muslim or non-Muslim.
2. This is not entirely true
3. No, this is not a justification at all.