To me, it seems fairly straightforward. Someone who commits suicide is suffering from a mental illness. Someone who is a suicide bomber is tragically misguided, and has a misplaced sense of religious fervor. And I almost want to say that the motivations of each are the opposite of the other - one is looking to end their life because they just can't bear living anymore; the other is looking to gain great rewards in the next life by making the ultimate sacrifice - which implies that their life is of value to them, though they choose to trade it in for a greater cause. Last I checked, religious fervor was not a mental illness. Granted, it would not surprise me if some suicide bombers were also suffering from mental illness, but that's just statistics. Get ten random people in a room, and, chances are, you will be able to say the same thing.