A good friend of mine teaches philosophy and ethics and supports the death penalty. I'm a former prosecutor* who opposes it, but he's convinced me that people can make reasonable arguments on both sides.

I can dispassionately list some of the arguments in favor if you seriously want to know what they are, to understand the other perspective.

The arguments on both sides generally fall into three areas, utilitarian, normative, and pragmatic.

Utilitarian arguments in favor of the death penalty claim that there's a deterrent for future crimes, that essentially society must choose between the death of a murderer or the death of future victims. Responses to this argument typically question the effectiveness of deterrence. Causation here is hotly debated, there are studies on both sides, and it strikes me as generally unsettled (and likely unknowable).

The normative arguments are harder to understand, because most people today instinctively gravitate towards liberal utilitarianism.

Normative supporters of the death penalty find it grossly immoral to suggest that we should allow some extreme criminals, say someone who serially tortures, rapes, and murders children, the future enjoyment of life's pleasures.

Take an extreme. Would it be just to set aside a special palace of pleasures for such a criminal, where they can live out their days enjoying everything they like (except for torturing children to death)?

No, at least for most people, that would seem odd. (A very strict hedonic utilitarian might argue that we should provide pleasure palaces to serial killers, but such a view strongly clashes with our intuitions.)

Extrapolating, should we allow heinous criminals to enjoy the feeling of the rising sun on spring days and the smell of freshly cut grass outside the prison yards? Should we allow them to enjoy the camaraderie of fellow prisoners they meet?

Why does lavishing pleasures on them in a special palace seem so morally odd and allowing these continued pleasures of life seem ok? For the normative supporter of the death penalty, it's not. After a certain line has been crossed, certain categories of especially heinous murders, the state has an obligation to ensure the criminal no longer enjoys life's pleasures.

Another normative argument is along the lines of the "worst crimes deserve the worst punishment." When you study crime for a while, it becomes pretty apparent that some crimes are not just more intense than others, some are so heinous they belong in an entirely separate class. It seems wrong to punish them in the same way that we punish theft or assault but just for a longer time, because the spectrum of inhumanity gets so extreme. They are different in kind not just in degree, and the punishment should be different in kind as well.

Pragmatic considerations arise for a variety of reasons. There are arguments about the cost of housing a prisoner for an entire life. Cost may seem a base concern, but if the system simply ignores cost, then it will be able to process fewer criminals, which means some serial criminals go free. (Death penalty skeptics will question the cost figures, noting that the appeals process is generally lengthy and expensive.) Another consideration is the impact on victim survivors of heinous crimes, who may know that the person who murdered their family and tried to kill them would like to escape and finish the job.

If we are unable to securely hold criminals who might break free and commit more crimes that raises other pragmatic concerns. Call it the Batman rule. Even though I oppose the death penalty, I think after the Joker escapes a few times to kill more civilians, after that Batman starts to become responsible for everyone the Joker kills.

Comics provide a straw hypothetical, but you don't have to go too far before drug kingpins like El Chapo are regularly escaping or buying their way out of prisons. We don't have escapees too often in the US, but a variation on this, criminals can commit crimes while locked up. If criminals repeatedly violently attack other criminals while locked up, and if we have exhausted all other measures of control, then at a certain point we cannot either house them or let them free while ensuring the safety of others. Since the state has a duty to protect innocents, in those cases I feel like the arguments get stronger.

For me, the arguments against the death penalty that I find most compelling come from understanding the impact of the process on living victims, who have to repeatedly attend appeals. They come from biases that emerge in the system (primarily against the indigent and mentally ill). I worry about misfires in the justice system that could lead to the execution of innocents. I feel like we can accomplish similar societal goals through incarceration, and if you can choose between two options where one has less killing, I think that option is generally superior. I also worry about whether any state should have the authority to take the lives of its citizens, bit of a libertarian critique.

I think there are some very bad reasons to support and some very bad reasons to oppose the death penalty that each get used quite a lot. I get the impression most people have a strong gut reaction on one side or the other and leave it at that, never really learning about the other side.

* Note: I never tried a case that would have involved a question of the death penalty, though, for what it's worth.