Where do you draw the line?

Two wrongs do not make a right. How do you decide who to kill and who not to? If they killed more than two people? Like where do you draw the line? There is no right or fair way to decide who to kill and who not to. It doesn't even make sense.
Also, everyone has a different religion and has different beliefs. Many people believe that before someone dies they need time to do soul searching and seek forgiveness for their actions before they are able to go to heaven. So why should anyone be able to play God and decide who should die and who should live. Especially before they've had that time.
And, what about the criminal's family? Isn't it shock enough to find out their loved one or someone they once knew has commited a really bad crime and is going to jail, but to find out they are being killed as well? That doesn't seem right either.
It must be hard on the people who have that job and have to take these people's lives also. That's why they got rid of it in so many places. I'm pretty sure it's not a good or peaceful death. It is impossible or at least it should be to decide where to draw the line when it comes to people's lives.