Leading up to the fight between Mace and Palpatine I doubt Mace realized just how skilled old Palpy was in the Dark Side and how close the Jedi were to losing everything.
It was only after Palps managed to kill several Jedi knights without breaking a sweat did Mace get the idea to kill him. I guess that took Mace over the edge from arrest to killing.
Mace wasn't evil. He, like the rest of the prequel Jedi, were just stupid.