We actually discussed this in one of my college courses. Bear with me, though. I graduated from college in 1999, and I'm totally going from memory. As I recall, a suicide killing of today are not all that different from the kamikaze pilots of World War II. In both instances, the person who takes their life as they kill others believes they are paying homage to their country or religious deity.
In the case of suicide killings that take place today, I believe that individuals to who do so believe that because they are making the ultimate sacrifice in laying down their life as they slay their enemy, they will receive a substantial reward in the after life.
Please note, this answer is based on a college discussion that took place over a decade ago. I could be way off. If someone else knows more about this subject than I do, please set the record straight.