I get the feeling that from your post you would on the normal side of things, sound very grounded and realistic in regards to what constitutes a relationship.
However, when you come on as ConfusedAndNEWLover.. You're making a bold statement right there perhaps hiding behind reality. Sounds to me like you did sleep with this male friend of yours and your either in denial or covering up big time, the Internet is a weird place, if he wants to see your phone, so too can he view what you write on line.
5 months is a long time to be broken up. If you were compatible enough irrespective you would not go 150 or more days without each other, you would have gotten back together way before that.
You are correct. What ever you or he did, whilst broken up is none of his or your business. Regardless of time.
Discussing killing someone and understanding that, is possession.
Wanting to confront this guy is possession.
Maybe even getting back with you was possession.
You are his possession, no one else was / is allowed to be with you, as he has been with you.
It's a bit more than trust and you have to ask why he doesn't trust. You haven't stated that he played these cards anywhere within the 2 year relationship rather it was "irrelevant" to the "issue now" that suggests that it wasn't about cheating or trust issues in the past, rather other factors.
He's immature irrespective as he can't handle that you had the right to do what ever you wanted over those 5 months as you two were not an item.
You have to consider why there was no contact for 5 months surely you can't believe that there was so much love you had to come back together again, rather perhaps, loneliness or familiarity.