[Smile-n-Win]

He didn´t kill anyone, he "just" helped those who did.

Quotation marks are right. "Just" helping murderers is just as bad as murder. This man absolutely deserves to die.

[Michael81Dus]

There´s a difference between pulling the trigger/flying the plane and standing at the side and shouting "do it!". While the first indeed committ the murder, the second supported his actions to a less extent than the actual killer. The second shouldn´t be punished the same.
The question why a state takes the right to kill people, can remain unanswered here. The only reason I know for capital punishment is that emotions calm down when the killer is dead, nothing more and nothing less. Not a christian attitude, I experienced by myself, when I supported the death for the snipers.