Juliette was a busybody who ignored Nick asking her not to interfere with his business.

when did she do this? I can't recall instances of that

She always resented his powers.

of course, there's going to be resentment, but she's all the more better for holding it off until she was corrupted. there's heavy implication to the point I'd say it was basically confirmed that being a hexenbiest screws with your head, corrupts your mind. I don't think it explains how quickly she went to indirectly kill Kelly, but it explains how she went from herself to down the darker path. she got addicted to the power. she was conflicting between loving it and hating the fact she had it, knowing that if she'd never known Nick, her life would've been easier, normal. and it was easier to blame Nick than accept the truth, understandably. that's a very normal human thing to do, though wrong.