About INTJ enemies - I think they're only bad enemies if they want to do something nasty - and if their tertiary Fi will let them. Frankly, I have no respect for revenge, and would not participate in it.
As for why villains are so often INTJs...I always figured it was because
1) Introvertes are weird, antisocial, uncooperative and therefore evil
2) Intuitives are strange, want to change structures that work just fine, and think on the 'global' scale
3) Thinkers are objective and do not understand the power of empathy, love or compassion
4) Judgers are manipulative and will bend the world to their will, perhaps by implementing complex long-term plans
Now, of course, that's all tongue in cheek - it hardly needs saying, but I don't think INTJs are really like that and I don't think the majority of non-INTJs think that INTJs are really like that. But, you can see how you could take some INTJ attributes and just warp them a little bit to get a fairly typical villain.