(a) Sometimes the wrong people get put in prison or solitary confinement for life too, which is effectively decades-long torture. It is arbitrary to draw the line at the death penalty, which generally causes less suffering. (Read statements from prisoners in lifelong solitary confinement.)
(b) It's not. Killing people that are evil is fine. Killing babies is wrong. You are intentionally simplifying the moral argument here to "killing people is wrong."
Maybe don't make prisons so awful that they constitute torture? That's very much a thing that we know how to do.
>Killing people that are evil is fine.
Who gets to decide what is evil? What happens when the definition of evil changes? Practically every genocide in history has been justified by that logic - they are evil, therefore we are justified in killing them. Other than vengeance, there is little benefit to state-sanctioned killing for the purposes of punishment; all state-sanctioned killing carries the risk of justifying other kinds of state-sanctioned killing, by removing the simple and clear moral foundation of what a legitimate state can or cannot do.
(b) It's not. Killing people that are evil is fine. Killing babies is wrong. You are intentionally simplifying the moral argument here to "killing people is wrong."