Tyrant, I realise that this is a real blindside, and you feel like you are somehow at fault...but you aren't.
Should'a, Could'a, Would'a: we ALL use those words, and we are all wrong! Your roommate made a decision for himself, and there isn't a single thing you could'a said or done to stop it, short of having him comitted for suspicion of "Being a danger to Himself or Others"; in short, a Psych 72 hour hold.
What would that accomplish? Maybe, just maybe some Headshrinker would have actually helped him, but most likely, he would have B.S.'d himself through the "hold" and then gone ahead and done himself...and maybe have taken you along for the ride. Other possibility: He'd have a supply of "Happiness Pills" prescribed, and slowly melted his pre-frontal lobes into syrup with prescription drugs.
You are RESPONSIBLE for that to which you are ABLE to RESPOND, not the whole world. Hard lesson, but vital.
First find out WHY John decided to end his life. You'll probably learn that there wasn't a thing you could have done to stop it...so stop beating yourself up for something that you could not have stopped, short of having John comitted.
It's easy to "see" things, like "the Signs" looking backward at an event, but it's bloody damn impossible to see those same "Signs" when you aren't specifically looking for Suicidal Ideations and Actings-out...and you weren't. It's easy to say this, because Pre-Suicidal stuff isn't all that obvious. >>>Was he suddenly acting happy, like he hadn't a care in the World? Was he giving gifts...usually stuff of his own...to friends and acquaintances? Was he spending every penny he had, playing and having fun, seemingly without a care in the world? Had he set a date when he would say "Everything's gonna be allright, then?"<<< These are all Pre-Suicide activities, and unless you have had the training to actually recognize them, you'll just shine them right over.
What he did was tragic, What he did was incredibly selfish (unless he was terminally ill), and What he did was stupid: Suicide ends chances to get better, forever.
Sometimes, Suicide is done to intentionally hurt others: like he has hurt you. That's why I say FIND OUT WHY!!!!
You say that Life got in your way...it does that, no fault to you. If it's important to you, it's not B.S....it's your life!
Before you do any more self-flagellation, get the facts; man! Don't assume, approach as if you know nothing about this death, and learn what went down from professionals. (Expect to talk to the Police a lot over the next few weeks) And foremost, remember it's not your fault, now or ever.
Bright Goddess grant you Peace and self-forgiveness...