Who's to say that he started off as a violent maniac? Chances are, he had a temper that he kept under wraps... and he entered the sport for a variety of reasons. Number one, he was good at it...secondly, it kept him in great shape. It brought wealth, fame, and the adoration of attractive women... if being a pre-school teacher could do that, he might've done that too.
His training gave him a deadly tool to use to vent his anger, for sure... but it may not've started out that way.
How many concussions did he have during the course of his career? I had 1 concussion (that was bad, but nowhere near being KO'd)...and let me tell you, even to this very day, I have a shortened fuse and occasional outbursts of anger or violence that even I can't understand why I would be upset (I also have ongoing problems with memory loss as well). I do a lot of work to keep myself calm, to recognize when I'm starting to spiral into an irrational outburst (they are almost gone, thankfully...). I'm not a violent person, never have been, and rarely ever have had to resort to physical violence... so fortunately, that was never something that happened when I had those irrational outbursts.
So...what if you *are* someone who makes use of violence on a regular basis, even if in a controlled and disciplined environment? You are literally out of control...out of your mind... in those cases.
MMA fighters take a lot of punches, kicks and other trauma to their heads. It's one of the controversial things about the sport... and it's a sure fire recipe for concussions of varying severity...all of which are cumulative when it comes to the dangerous side effects. All the people that cheer when a KO is delivered? They are cheering for permanent brain damage.
With that damage comes everything from decreased motor or vocal skills to complete personality changes. Hopefully most can avoid that fate, but it is a serious issue in MMA...one that probably hasn't been studied much in that sport (although other sports have now been paying a lot of attention to it).
Now, couple that with the potential abuse of drugs, alcohol and performance enhancers... and you have a lethal cocktail. Partying aside, those things are also an effective diversion from the horror of feeling like you're losing control of your mind and body (which is what happens with a personality change or irrational violent outburst). Does anyone actually try to counsel MMA fighters on their fears surrounding the sport? The unspoken damage to their minds that they hold onto in silence? I very much doubt it, as it's an ultra alpha sport where macho bravado is king... and anything less than that is weakness. Stoicism kills.
