As Maxtro said our species still hasn't evolved beyond an intrinsic affinity for violence. This is evidenced by the world spending trillions of dollars on more efficient ways of killing ourselves, while there are numerous other, more pressing problems are staring us in the face. The popularity of contact/collision sports like MMA and football is simply a reflection of what our society values.
Most UFC fighters are not douche bags... think about how often you hear about UFC fighers getting into DUIs, gun charges, domestic violence, etc compared to NFL guys. I'm both a huge NFL and MMA fan, so I'm not pitting one against the other, simply stating a fact. Fighters are pro athletes, which means they have to train day in and day out to achieve victories and maintain winning records. Mentally unstable individuals would probably lack the discipline and mental strength to follow through on a pre-fight training camp.
There are many well known UFC fighters like Shane Carwin who are quite well educated. Guys like GSP are extremely marketable because they are "nice guys"... until you're in the ring w/ them. In many ways I think the UFC does a great job of marketing "****y bad boys" against your classic "good guy" (think Dan Hardy vs. GSP). You gotta respect Dana White and the UFC for their superb marketing skills, as the interest in MMA and UFC has grown very quickly over the last decade.
