I might be for it in theory in very limited cases, but in practice, even if for the sake of argument capital punishment would be an effective deterrent in those cases or bring victims any "closure" without diminishing their own humanity, etc., the problems that remains are:

Justice is ineffective if it's not swift. The way capital cases normally play out, there's 10 to 30 years between the crime and the execution.

And justice is pyrrhic and beside the point if it costs society more than just warehousing someone for life. We spend tens of millions litigating / blocking / advocating for the sentence to be carried out, even when it IS finally issued. Even after the execution there may be lawsuits against the state for botched executions, which are an increasing problem, for allegations of cruel and unusual punishment, etc.

So even if I were rabidly in favor of the death penalty, I wouldn't favor it

I'm against it completely in all cases. Killing a person is wrong period.