I truly believe that spending your life in solitary confinement with no access to anything but your thoughts would be FAR worse than putting them to death. Let them stew in what they have done for as long as possible. It cost less than execution thanks to the ridiculous appeal process afforded to death row inmates.

In an attempt to answer your questions: 1.a. see above 1.b. regardless of reason, assassination has generally turned out badly. I love the time travel joke about killing Hitler but a different guy does the same thing. 2. The one to be executed losses their right to appeal and prove their innocence, as we know there have been many people wrongfully executed. They murderer of said death row inmate just becomes one. Loes loes 3.a.currently the president can legally have his political rival assassinated, or so I'm told, and this is far too much power for anyone to weild. 3.b. I don't really see the death penalty as just 4. Yes every case needs to be evaluated separately. Assassinating a brutal dictator to bring people democracy, probably could have been done without murder. A woman being assaulted who accidentally shoots and kills her attacker, totally justified, panic reaction, give them therapy. A psycho who totrues, kills and eats babies, death by 1000 cuts.

I hope this helps, I hope my answers are sufficient and understandable. I'm happy to clarify if needed