I don't think there's
logic to really be considered. We are, after all, discussing fictional characters. Holden says it best:
https://youtu.be/0nVGCVlCzL0
I completely understand your opinion that it was something, you felt, Heather needed to do. And it's not that I disagree with your opinion. It's yours, and that's cool.
I guess, for me - when I think of Heather, I never saw her (prior to this) as someone so ruthless. I mean, even when they first found Logan, she was defensive over him - wanting to utterly protect him, even at the risk of her own health. She had always been written as someone who was very gentle and caring. So, her blasting Snowbird - I get why she did it - but it seemed, to me, out of character. It'd like watching this same situation go down and thinking that Captain America, or Thor, or Hawkeye would kill a fellow Avenger in this same situation. I don't think they would.
So I get why the writer did it. He needed to shuffle out the "old school" Alpha Flight (I still believe this was his plan all along) to usher in a new set of characters to take over the book.
I mean, take out the fact that it's Snowbird. Had Heather blasted - I don't know, Earthmover - I probably wouldn't have been annoyed back then - but I'd still be under the belief that it seemed out of character for her.
Circling back to the original post - Heather killing her cousin.
As we stated - under mind control, Marvel always excuses. Prime example, Captain America (the boy scout of all boy scouts) holds no apparent grudge towards Winter Soldier (Bucky) who killed Nomad (Jack Monroe) while "under the influence" of someone's mental manipulation. So if Cap can "forgive" someone, it seems like anyone in Marvel can.
And the story has moved on past that, in World War Wendigo, where we see Heather is apparently no longer under the control of Unity, and "back" to herself.
As to why Puck and Wolverine didn't do anything - well, since she's cured - who knows. She may have gotten help from them (off panel). If not, I could see that they may not have known. Puck was busy palling around with his new team that he was on X-Force, while Wolverine was busy being an Avenger, an Uncanny Avenger, a Secret Avenger, X-Men, X-Force, Uncanny X-Force, and Spider-Man's side kick, and whatever other title Marvel has - so Wolverine was probably entirely too busy himself dealing with everything from every book he's in.