Resisting the urge to maim

Tom is lucky to be alive.
You know how some people become first time parents and there's like this amazing flowering of personal growth because having a child has pulled them outside of their own minds and you're just like "WOW, you're actually a worthwhile human being! Who knew?!?" Okay, now think of the opposite of that.
He's fucking lucky to be alive. It's a wonder his body wasn't found in a ditch on the way here (here being Kentucky).
 I am sympathetic, because some of his behaviour has to do with the shitty relationship he was in before. She was a piece of work...in the bad way. She fucked him up, over, and under. She was also a skunt. BUT...if my brainmath is correct, it's been over two years now. His new wife (the wonderful Wendy who has her own problems from a previous relationship but is a LOT more aware of that) is NOTHING like the old one. Serious upgrade. His family agrees. I would submit that continuing to react to her like she was his ex is a fucking insult.
IN ANY CASE. While some of his reactions are because of the former and I can understand that...there's a line. And he keeps crossing it.
He also is failing at parenting because he can't stop panicking. Here he is with a teething baby in his arms, a crying teething baby, and what is his reaction? YELL. When his wife (who has two much older children and thus experience) asks him to stop yelling? He continues yelling that he's sorry, he's not mad, he just doesn't know what to do. At which point she tells him AGAIN to stop yelling. Somehow he can't seem to clue in that a baby doesn't understand that he's not yelling because he's mad, a baby just knows that she's in pain and now her daddy is being LOUD AND YELLY. He also can't understand why his defensive behaviour and inability to take constructive criticism might piss wifey off. Tempers fray. Muchly. and now we segue.
I was asked to tag along so I could help Tom bring home malted shakes for everyone. Fine. His adorkable niece ends up tagging along. Great. He goes around supposedly making a list of what malts people want so he's all organized (theoretically). Reasonable. He keeps repeatedly asking me if I'm going and when I say "sure" snaps at me to get ready to go...repeat this step over a period of twenty minutes to half an hour. Eyetwitch. Niece and I have been goofing off and harassing each other all day, he asks us to cut it out in the car, as this is a reasonable request we comply. We get to the store and get in line. He has forgotten the list in the car. Niece and I step aside and goof off (she was bopping my nose and trying to keep me from bopping hers). He comes back in and orders. Great. We continue goofing off. He bites our fucking heads off. At this point I am thinking "okay, that's legit, I can understand how the immaturity would get annoying. I will accept this even though his tone is completely out of line." He screws up his order. People get in line behind us. He screws up his order again. And again. And again. We (niece and I) try to help keep him calm and figuring out the order. This is not a rewarding experience. After umpteen order changes (I felt so bad for the guys working and the people in line behind him) we finally end up with an 8 malt order. 5 chocolate, 2 cherry, and a cookie dough. If you couldn't guess I KEPT HEARING THIS FUCKING ORDER. While talking to us he keeps screwing up what's in his order. So when the guy says "Here's cookie dough and 2 chocolate" and tom responds with a comment that sound an awful lot like he thinks that's all the chocolate he's getting I let loose with a flippant and dramatic "BUT WAIT, there's more!" He starts going off on me, and as it seems like it has to do with the order takers I think it's reasonable. So I politely say that I'm sorry, that's not what I intended I just meant that there were more chocolate malts coming. HE FLIPS THE FUCK OUT AND ORDERS ME TO GO STAND OUTSIDE AND WAIT BY HIS CAR. He's six fucking years older than me. I won't let my own MOTHER talk to me in that tone, and she went through three days of labour for me! This does not fucking fly. I don't know if my eye was twitching, but I must have had a murderous gleam in my eye as I stared him down. I think I said something like "Oh, WE can TAKE this outside." He starts apologizing and trying to blame his alcoholic consumption of the night before, or the fact that his wife is cranky (because he wouldn't stop fucking yelling around their teething and vomiting 6 month old baby), or that they didn't get enough sleep OR OR OR. All that's going through my head is how many times he has lashed out at me over the past month and apologized for it. People don't realize it, but I have a pretty potent temper. I made the decision to leave before I lost my shit in public. As I turn and go towards the door he says "No, you don't have to wait outside." I informed him that I thought it would be a good idea and leave to go pace behind the car. I think it made him uncomfortable because he ended up bringing his keys out to the car. When he and Niece come out with the order he apologizes, stating it's nothing personal. "It never is," I reply. I also ratted him out to his wife (and feel no guilt about that) and he apologized again.
I should add that the man does have PTSD. Which is why I have endured a LOT more bullshit from him than I would normally consider acceptable. A LOT. Because the asshole thinks it's okay to dish out teasing and shit on me, but if I reciprocate (or, heaven forbid, aid his WIFE in teasing or shenanigans) he flips the fuck out. I keep telling him "Don't dish it if you can't take it" and he keeps insisting he can.
Bullshit.
You can just IMAGINE how much I'm looking forward to the trip back :/