Board Thread:General Discussion/@comment-26200221-20160223142644/@comment-34326521-20160311063909

Let's start listing reasons then, to see if I'm right:

Ben is a teenager. They're notoriously unstable, due to physical and hormonal changes. It's not their fault; it's just how biology works.

The lack of experience in dealing with such extreme anger would make Ben snap in a big way. It also explains why it took more for Ben 10,000 to snap.

Ben was indirectly (maybe even directly, if you consider young Ben in the episode as being the same) for Kevin returning to his evil ways. His guilt would push him to fix the problem.

Ben has a track record of blaming other people for his problems (remember how he met Kevin?). It's not unusual for people to blame things they do on other people. It is, in fact, a very common trait.

Ben knew Kevin (the non-power-crazed Kevin) would rather die than be a monster.

Ben had had his family and friends hurt by Kevin in the original series.

Ben had seen what Kevin does when he gets "ultimate power".