Grimlock ♥ Ultra Magnus

Matt left Heather after she begged him to stay and said she was afraid of what would happen if he left.   She said she was lonely with anyone other than him.   Keep in mind they almost got married, and he destroyed her beloved, and dead, father's business so it wouldn't be competition for her time since she was running it.   (He only did this after he put her down, telling her she couldn't run the business, and she should marry him instead in an effort to get her to resign and be his wife.   This was also right after Elektra died, so, yes, Matt was at a very, very low point.)

Matt was going to intervene in a domestic dispute going over to Heather's but thought she was in immediate trouble.   The woman was killed by the time Matt got back to said domestic dispute.   He lashed out at Heather when she called him again, saying that if' he'd gone to the woman's help, she'd still be alive. 

She calls him over and over, and then she calls Matt's business partner, Foggy.   Foggy's at Heather's, calls Matt, and tells him to come over, where it looks like she hanged herself.   He wanders around, saying if only I had then she would still be alive. 

He doesn't realize that things don't add up: the smell of something that shouldn't have been there.  He goes back, investigates, realizes that there are cigarette butts - Heather hated cigarette smoke - and that her safe is empty and decides she'd been murdered, so, y'know, he shouldn't have felt guilty.   Because, y'know, he didn't leave an opening for the  murder to go in by leaving her or anything.   Also, this clearly fixes the shitty way he treated her, another reason he'd been feeling guilty, because it didn't lead her to commit suicide. 

Also, Matt feels guilty over things he has no control over.    He'd still totally be feeling guilty even if his ex-fiancee had been murdered, no matter if he'd been on the other side of the world.   I'm starting to feel that Dennis O'Neil just doesn't get DD.