he needed Anakin to get involved and be the one to betray Mace so that he would feel as though there was no going back. Had Sidious destroyed Mace (if he could), then Sidious would have dead Jedi to account for and no assurance that Anakin would join him. this is one of the most common misinterpretations i see in this discussion. all sidious needs is for anakin to NOT get involved. by doing nothing to prevent mace from getting killed, anakin is guilty and there's no going back. so... as long as sidious knows anakin will do nothing to help mace (which he witnesses himself) it's the best strategy to kill mace then and there. why wouldn't he? it's clear that anakin won't intervene. mission accomplished. no need for this faking crap which could result in his death, when he already has what he wants. the only explanation can be that sidious was indeed overpowered and needed anakin to help him out.