I'am against death penalty even in extreme cases

Why?

1)
What if you kill just one innocent person? Is that excusible? It's not like the US Justice system is 100% correct all the time and innocent people do get arrested.
this reason alone says everything.

2) if the state kills people who killed other, then the state is not better than the criminal. It's a moral degradation to simple revenge. Revenge is not the main idea behind justice. There are other better ways to punish.

3) in some countries the death penalty is some kind of social issue too. For instance in the VS: poor people can not spend much money to have a good lawyer. It's some kind of class-justice. Also there's a higher number of blacks that get the death penalty then white even when the amount of crimes is the same... so there's injustice in the system. If you kill people with that injust system as reference then you have a problem.