Originally Posted by IlliferThePenniless
Chime in here guys, I don't really have any citations to hand, but I definitely have the impression that Varys fed Aerys' paranoia and probably intentionally (though, strangely, he did, allegedly, beg the king not to open his gates for Tywin). Am I totally making that up?
I think you are correct that he fed Aerys' fears. In which case if he really is out to better the realm what better way to do it at that time than to remove Aerys, which would have given the throne to Rheagar! Yet loyalty to the Targs would have meant Aerys over Robert, so don't let Tywin in!