No, killing another sentient being is never justified. Human rights for everyone were declared for a reason.
First of all, human justice is never perfect, who knows how many people, even in the US, have been killed but were innocent over the decades. Better ten guilty men go free than one innocent man gets hung.
The "enough evidence" argument can not have a clearly defined lower limit, so it can become a slippery slope.
Then there is the escalation problem: Oh, crime X sure was bad - death penalty! But then the next one comes alone and does X - but with children, triggering the "zomg think of the children" effect, or kills X + 1 persons. Soon you'll want to kill and torture them to compensate.
Also, people change after years in jail, a bit like the ship of theseus does. While the person who did the killing 10 years ago might have "deserved" it, the person now has changed quite a lot. Remember for example the guy in California, Stanley Williams, who was involved in murder in his teenage gang years, but later became a five time Nobel Peace Prize nominee but was nonetheless killed.
And using the death penalty is a final confirmation that violence is a means to an end, especially when you are killing a defenseless person. That has widespread implications for all walks of life.
And on a final note, it has repeatedly been shown that the death penalty does not deter criminals.