I would completely agree with mindcrime and Cem Kalyoncu.
Take into account that passive agressiveness for example is more difficult to detect (irony, black humour, sarcasm likewise)
Although another head start could be to think out of the box:
Happens by chance that I have lying around a book concerning violent-free communication. So probably your best start could be to talk with linguists about violence in language and start from there. or just make some reviews about how linguists or psychlogists detect violence in language (susprise: It's probably quite complex)
Nevertheless: I don't think you need real AI, a blacklist of words and expressions together with some pattern detection for expressions could be precise enough for the beginning.
Then for all the expressions, words, etc. you could add a bayesian network for the learning part, which works with probablities (like e.g some email spam filters)
Search for example for "Naive Bayes spam filtering"
This should be pretty much enough to have a good start, so in strict sense, you don't need real AI here, just BusinessIntelligence and probability calculations.