actually, I'd be more likely to say that Lelouch would be the likeliest one to go crazy and turn into a maniac... We've seen what out of control Geasses can do to a person, and now Lelouch might very well have to deal with losing his best friend on top of already having killed two of his cousins (who weren't even involved in his mother's death) and his half brother (who probably did deserve it), while having to lead a crippled and dispirited resistance group. Plus, it's pretty easy to see what kind of plot event would lead Lelouch to completely lose it.

And then you'll have a super smart, super powered, military and organizational genius who has no problems killing upwards of thousands to see his goals met (he already has this trait). And then you'll also have a very proud Emperor of Britannia...

but, since bishounen main character can never = the clear cut bad guy, it's more likely that Lelouch loses it only a little, if at all.