He's not a fundamentally hostile person at least, he doesn't come off that way on television. With Letterman, contempt for his guests is often palpable. Letterman is so fearful of failure that at the slightest lull in the show, he may expose a guest to any form of meanness that could get a laugh. Then again, insecurity and hostility are pretty much two sides of the same coin to begin with. There has been some improvement lately.
Letterman often seems to work at being kinder and gentler to guests.
The problem is that it's such obviously hard work. True change comes from within. Still, there's real cause for hope. Letterman's show has seen a slight ratings upturn this season after some format changes. Maybe all he needs is the sort of genuine softening of outlook that quintuple bypasses and other brushes with death have been known to induce.