Actually, at the time, Palpatine(sidious) was still much stronger than anakin, yet mace could have killed sidious eventually if he kept using lightsaber form 7: vaapad. This lightsaber form taps in to the darkside,which makes the user of this form almost unstopable. To use this form, you have to master several other lightsaber forms. mace is a master of vaapad. So i don't think Anakin would have been strong enough to kill Mace Windu, using the darkside or not.