what drives Trump's habit of awful tweets

I think Trump's habit of awful tweets is being driven by his craving for popularity. The same goes for a lot of the awful things he says out loud. As this article by Kevin Drum of Mother Jones points out, the intended audience is his fanbase -- it's just a nasty side effect that the rest of the world is horrified and offended, or that our overseas troops are endangered.

I think what Trump has done is found an audience that is willing to give him the cheers and praise he wants, and they're the people that nobody else wanted cheers from, so he's kind of got them monopolized. I'm talking about the racists and xenophobes and misogynists and other deplorables -- the sort of people who actually cheer for the destruction of the environment, just because they hate environmentalists; people who condone violence against women and applaud police brutality against minorities; people who despise diplomacy as an alternative to violence, and feel there should be no such thing as international law. Trump has found that by egging on the worst tendencies of America's worst citizens, in ways that few others are willing to stoop low enough to do, he can finally get that long-sought popularity. That he's pushing the country toward fascism in the process doesn't worry him at all... after all, if it did become fascist, that just neans he could have even bigger adoring crowds, and to him, that's the important matter at stake -- the one we should all be talking about first.