Thats not true.
He was a boy crying for his mother.
He knew he was diff. from other.
And he found out they abused of HIS MOTHER !!
Remember the experiment in the tube (those weird monster) ?
Thats when he got mad and took Jenova out !

Tell me how would you feel if someone abused of your mother and would use her as a lab rat ?
And what would you do if you had the power to make them pay ?
Thats how it started.