Fiancé has been arrested for harrassing his ex

Context though she has stopped him seeing his kids, the thing is though I know he’s been going through this. He told me. The harassment charge was because he had left presents for his daughters during birthdays and Christmas, but he sent her a dubious txt when we went away and he posted pictures that she looked at. When she looked at them he went nuts, then last month police knocking on my door at 5am and he got charged. I backed him trauma if not seeing his girls etc. however yesterday comes in saying he wants to kill her because she had been on holiday with her new partner. He proposed to me Xmas day was lovely proposal, so why is he going nuts that she went away with her new partner on their own. I am in massive fear that I am an easy rebound for him, i have a lot to loose if we get married and it doesn’t work out. I love him but I am in this turmoil of is it my misinterpretation or is his reaction to what is to do with her and not his girls out of proportion. I would appreciate outside views and don’t want to talk to my family - we’ve just sent wedding invites!