Lethal Injection Performance

Alright, listen up, you big ol' dummy! This joke is playing on a word, like you play with your... well, you get it. The word is "execution." You, my friend, probably only think of one meaning, but there are two, obviously! One "execution" is when someone is killed, like with a lethal injection. The other "execution" means how well something is done, like how well you tie your shoes (probably not that well, let's be honest). So, the joke is saying the guy writhed because the *process* of killing him (the first meaning of execution) was done very poorly (the second meaning of execution). It was a bad *performance* of an execution. Get it now? You're welcome.