Usually, no one is around if the bomb went off before it was meant to.
I find it impossible to believe that someone who is depressed to the point of committing suicide is going to invest time and effort in making a bomb. It just isn't human nature. And for what, to blow himself up sitting on a bench with practically no witnesses? Making a bomb doesn't indicate someone who is depressed. It indicates someone who is angry and has a plan.
The only part of the depressed angle I can buy is that it could make someone vulnerable to being brainwashed and manipulated. Someone depressed and unsure of his direction in life, as Hinrichs has been described, would be a perfect target for recruitment and programming by Islamic handlers looking for someone to do their dirty work for them. Especially in a university environment. All the elements are there for such a scenario. And with everything that has gone on in Norman, I'd say the tinfoil hat should be standard equipment when looking at this.
I think you have the right answer -- he was vulnerable and the vulchers took the opportunity to convert him. Obvious that he and his Dad were not close.