Vayne isn't such a bad guy ?!?!?
Killing two brothers and his father aren't bad enough for you ? How about killing people who have just surrendered ? How about driving the Bahamut to Rabanastre to obliterate the city ?

Face it, Vayne lost it. He might have started with good intentions but in the end he lost it.

Which brings me to the point that XII is badly written. Vayne is shown to us as a guy who wants to be fair and nice to Rabanastre and wants to free humanity of the Occuria then - all of a sudden - he turns into a cartoon villain madman bent on genocide. Like it was time to have a villain and he was handy. Not believable at all.