First, most of these quotes are from the movie. As this is the novel thread, I would have thought those don't count.
Second, we all have the capacity for evil. We're humans, and none of us are 100% good or 100% bad. Jack Torrance was a deeply scarred man who, as I said on my blog, wants desperately to be a good husband and father, or, more accurately, wants to want it.
The problem is, for too long he wasn't willing to get the help he needed in order to be that man. He was just on the cusp of it being too late before that realization finally hit him, and he still did the stupid thing and went cold turkey with no professional help at all.
Both before and after he got on the wagon, he made excuses for himself, always telling himself that it was okay to just slip a little, just have one drink, just move the debate timer ahead a little, for George's own good, etc., etc.
I don't accept that he was "evil" before the hotel got into him, but flawed? Oh, yes. As are we all. This is why I love the novel and why Jack Torrance might very well be one of King's best characters.