I think going out like a punk was getting shot in the back by Clone Troopers, getting killed off in a way that is pivotal to the story is not. I think it was clear that Palpatine was letting Mace win and picked the exact moment to look weak just as Anakin was coming in the room. Further evidence is that Palpatine seems to be toying with Yoda as well and not taking him seriously at all almost like he could take him out anytime if he wanted. I also thought that his face always looked like that and the Chancelor image was a mask and he did not need to hide anymore since his plan was now underway to become emperor.