The reasons I'm against the death penalty... (in the US)
1) It is unevenly applied when race is factored in (yes, weighting for percentages of crimes committed). A black man is six times more likely than a white man to be executed for the same crime. Six times.
2) One in seven death row inmates is innocent, if he is executed before that is discovered, then no matter how sorry the state is, he's still dead.
3) As revenge, state-sponsored or no, is reprehensible, the only possible value of the death penalty is as a deterrent. Lethal injections won't deter anyone. Executions like this one performed in Iran are what the death penalty should be like. Chalk that one up as a point for the Iranians, assuming this guy was actually guilty. To be a deterrent, executions need to be brutal in the extreme, painful in the extreme, and public in the extreme. Roman-style impalings, hanging (by lifting, not the 'short drop, sudden stop' method), and similar techniques should be used in preference to 'humane' methods. The whole point of execution is that the condemned is less than human, doesn't deserve to live, and is going to serve as an example to others of what society does to people that do what he did. A quick merciful death is the last thing society should grant to someone that it intends to serve as an example.
I would support the death penalty in the US if solid, non-circumstantial evidence, first-hand, unimpeachable eye-witness testimony, or an unforced confession were the only means of securing it, it was sought in every case of the appropriate type regardless of race or other factors, and it was revoltingly brutal, televised on every channel, and done at noon so everyone was awake to watch it, and rebroadcast later that night so night shift workers could watch it when they woke up. Short of those changes, I will oppose it as useless and unfair.