Don't you think he must have had a car-bomb in mind for the ammonium nitrate?
Heh, this guy sure wanted to make sure he'd be dead after committing suicide, eh? No slashed wrists, sleeping pills, or even a bullet in the head for him. No siree. Why take any chances on surviving when you can use TATP or AN? We can be glad he didn't have access to a dirty nuke, to make sure he had absolutely no chance of surviving a suicide attempt.