Why? is it because he's a crazy man or has he got an evil plan? Also in B/W2 why does he return after being previously defeated in B/W?
Ghetsis basically wanted Pokemon to be free, and not be used like war machines and be controlled by Humans. He basically thought that Pokemon were suffering by Battling(despite the fact that Team Plasma used theirs for Battle), even though the Pokemon actually enjoy it. Your Second Q can be answered once BW2 is released on Sunday.
He really wants to use Poke'mon to rule the world, I think.
The "people abusing Poke'mon" thing is an excuse.
He raised N to believe that people are horrible, and Poke'mon are good.
That way, N would support him when Ghetsis took Poke'mon away from humans.
As for the last part of you question... Maybe he is so ambitious he will keep trying.
To get the full story, you would have to play both games, and possibly ask The Poke'mon Company why they made it that way.
So that only he could have them, which would make him the most powerful man in the Unova region and eventually the world.
So if he tries to take over the world, trainers dont have their Pokemon to fight back against Team Plasma.