Were suicide bombers suicidal?

Whenever I hear about a terrorist attack, where a suicide bomber has blown himself up, I wonder about that person’s mental state (prior to the blast).
Were they (in)sane, were they severely depressed are there maybe depressed members of terrorist groups that volunteer for said acts, totally brain washed, or something else?
Does it matter for the next question I have: are they first and foremost murderers, that lose their live in the action, or are they people that take their own life, taking other people’s life with them?
Could researching the motives and/or mental state of said persons be beneficial for the society that could potentially suffer from these terrorists?
Nota that I don’t sympathize with these suicide bombers (obviously); I am curious from a psychological perspective.