It's sure as heck true that having suicidal tendencies and being depressed (if he was) doesn't rule out by any means that he was attempting to take some others with him.
That's why Boren's running around spouting off about how depressed this kid was and how this was 'just a suicide' was such an obvious smokescreen. I can't get over how utterly ludicrous that was, and how clearly it displays his enormous arrogance for him to think that people are going to believe that he could possibly know this kid's mindset and intentions.
I'd suppose that it's reasonable to assume that the psychology of most suicide bombers is off-center, and that many or most are people whose mindset makes them prone to commit suicide anyway. As you say, happy, well-adjusted people probably aren't willing to make that sacrifice, regardless of how radical their worldview is.