Because the fans need Pete to be demonized in their eyes, to justify their deep-seated hatred of him and daring to interfere with Sam/JackOMGOTP! *cough*

Because his being pretty bland just isn't good enough for most folks. ;)

Thus, the path of least resistence is to make him physically abusive, because that's easy to write, visulaize, and hate.

Jack as the Off-White Knight has been covered amply already =)

J