I actually don't find it that hard to believe. All of our thoughts, behavior, and personality traits are dictated by neurochemicals, altering their amounts or ratios can have profound effects on a person (think of drug addicts).
Trauma to areas of the brain can most definitely cause changes in a person's thoughts, which obviously leads to changes in behavior.
I have suffered 3 very serious head injuries, and I know that it changed me a lot. People have said that I am noticeably different now, but this was without their knowledge of the events. They attributed my changes to growing older and having more life experiences (which obviously have an effect as well). I think my different state of mind is a combination of many factors, but the injuries definitely contributed to them.
Just the damage to my pituitary gland has changed the level of a number of hormones in my body, and those alone have led to changes in behavior.
The mind is so complex, it really fascinates me. I have always been intrigued by what makes people tick, the hidden determinants of human behavior.