Who can kill the people?

One of the my best friend's aunt was killed by someone who wanted to rob her. I didn't want to express my point of view about death penalty at that time. Several days later I went to a bar with him, I actually didn't wanna go because I didn't know what to say. After talking for some time he ended up crying like a baby and said to me; that he didn't want to see the killer's face in court, and if could he would kill him.I used to think that death penalty was not the right thing. I used to feel that no one has the right to kill anybody...
But I couldn't tell that to my friend who's aunt was just murdered. I do think that people who have commit murder should pay for their acts. On the other side, we have to consider the people who is accused and they are innocent.

But how could i say that opinion to my friend?I didn't have a that kind of experience I think thus i can think that system without any anger feeling. I never tell to him about that opinion but i don't think he is wrong I think he is right not only him but also all of sufferer are right.
 However we have to think about false charge too.


In fact my country has ５３people who are criminal condemned to death now.May be some of you guys think its so many however my country`s rate about killing is very low like 1097 in year of 2011.And also even some one killed guy if less than one person it never be death penalty.So Japanese government is killing people who killed people instead of guys like my friend.Is it wrong?right? who knows.

Fact box: Got from Wikipedia and Japanese news paper "ASAHI news paper"