As dope as it was seeing Mace beat the Palpatine down, I figured he was playing possum, knowing Anakin would come and save him, thus turning to the dark side in the process.

He even set it up by telling him to kill Dooku in the beginning, and making him feel guilty about killing someone who should be taken alive because it's not the "Jedi way." So by cutting of Mace's arm, he was in a messed-up way, trying to stay true to the Jedi.

Yep, Palpatine playing them all like a fucking piano.