It's the result of a decades-long lawfare campaign by anti-death penalty activists. The more protracted and expensive it is to carry out the death penalty, the easier it is to argue for abolishing it on the practical grounds of cost rather than convincing Americans of the ethical case. A rather messed up byproduct of this is cases like the one you highlight, where the convicted person is left on death row for decades as they make hail-mary appeals.

> but what does society gain from ruining his life in turn, to the point of what feels like mental torture: Being locked up for such a long time, all the while knowing that you will eventually just be executed.

From what I gather on Americans' comments about this over the years it's mostly about "not spending taxpayer money" to house, feed and take care of criminals that received a death penalty.

I don't know how true this argument can be given all the costs over decades associated with a death penalty judgment (appeals, preparation for death row, maintaining death rows, etc.)

Quick edit after reading the thread a bit more, an example of what I mentioned: https://news.ycombinator.com/item?id=26900987

The death penalty used to be much faster. But through a series of laws and court rulings starting in the 1970's, they decided that the case has to go through a super long sequence of appeals and court proceedings, with the intent of making doubly triply extra sure we're not executing innocent people.