Mace Windu couldn't kill Sidious
The prophecy stated that the chosen one would destroy the Sith and bring the force back into balance. If Mace killed Sidious before Anakin arrived, the prophecy would be untrue. Its almost that fate saved Sidious from dying at the hands of Mace. Mace could under no circumstances kill Sidious. It just could not possibly happen.
Well, in all actuality, I believe Mace could have. But it's like cheating death. Just because one is capable of doing something, doesn't mean one actually DOES it.
Maybe Mace could have killed Palpatine. But obviously, it was Anakin who was MEANT to. So, the Force worked it's mystic vodoo, and made sure that it was Mace who died - instead of Palpatine.
Of course, we're not all sure if Palpatine was toying with Mace or not. Who knows?
nah. palpy woulnd't let an idiot like mace off him. if anakin tripped and sprained his ankle or something on the way to his chambers, palpy would've just blasted mace outta the window in the end anyway. if you were a sith lord, would you let samuel l. jackson stop you? christ, you'd never hear the end of it.