There is a lot of stuff already in this thread, and while I did my best to scan over it all, I apologize if this has already been said: I do not think Mace Windu ever really planned on arresting Palpatine. I think it is still canonical that Mace leaned a little closer to the dark side than most, so he probably knew more than most the type of threat Palpatine presented. Furthermore, Mace was always very harsh and severe. In his mind, anyone who used the dark side probably deserved to die if they used it to harm others (that is just an assumption on my part based on his character). Anyways, my main point is that Mace probably went to go "arrest" Palpatine as a formality. Mace probably knew that Palpatine would not truly surrender, but he had to offer him the chance so that killing him would be justified. Kind of like how in a hostage situation they always offer the person a chance to surrender. They almost never do, but just in case, they have to offer. With that said, I personally feel like killing Palpatine was a justified move from Mace's perspective. Unlike most war criminals, Palpatine posed such a threat and was so evil that imprisoning him (no matter where) would have been pointless. Mace understood just how far Palpatine's reach was. Even with video and audio proof and a witness (of which the Jedi had only one), there is no way that Palpatine would have been convicted of his crimes. And even if he was, there is no way he would stay in prison for any amount of time. Palpatine was just too powerful. It can almost be argued that it was worth betraying the Jedi to prevent such an evil from remaining in the galaxy. As a side note, it cannot be argued that Mace was killing an unarmed person. Palpatine was still capable of multiple force attacks, and Mace (and Anakin) knew that. Another side note, I personally feel like Palpatine forfeited the right to a peaceful surrender by attacking initially. Any "surrenders" after that could honestly be assumed to be deception. Lastly, I do not believe that Mace's attempt at killing Palpatine touched Anakin in any way on a moral level. Anakin simply did not want to see his only tool for saving Padmé destroyed. This is perhaps most clearly seen by Anakin's slaughtering of the young ones at the Jedi temple almost immediately after "saving" Palpatine. Also, Anakin had to walk past the three mutilated Jedi on his way in to Palpatine's office. I am pretty sure he could tell Palpatine did not kill them in self-defense.