Found this in my drafts, for some reason I just didn't post it? I don't even know what I was on when I wrote this but...Yay... I guess?
I just had a theory about the stabbing scene: Melanie wasn't upset he stabbed her instead of himself because it was a silent agreement (note that she asks if they're gone now; she likely realized about the vow as Halligan stabbed her/yelled "The vow is broken").
There is the general opinion outside the fandom that this was done out of selfishness. I would like to argue the opposite. Here's why: Halligan doesn't have great self-preservation instincts, and doesn't seem to have much of a reason to avoid stabbing himself. Remember that Halligan doesn't seem to mind crossing the bridge on a very unstable board, close to falling into the pits of nowhere. Halligan is very reckless. Self-serving, sure, to an extent, but not that much; I mean, he really doesn't have much of a reason to be following through with the case at this point to begin with, remember.
Say what you will about Halligan, but he also listens to Melanie when she tells him something, so it's likely that when she said "Don't you ever do that to me again, Brent!" at Twelve Bridges, he interpreted the "that" to mean "put yourself in danger", and thus decided to TRY to avoid it as much as possible. When Melanie gets upset about what he did at the mansion when he was brainwashed, she only basically says "Don't you ever hit me like that again" if I remember correctly, therefore, stabbing her is...actually fair game in the situation at the end of the game.
Halligan took her words to heart. He doesn't care about the risk to him, he was worried about upsetting her by doing so. Being the way he is, to him there was no sense in thinking about this the standard "self-sacrificial way" here, especially after she said that herself. Melanie is pretty understanding of the Halligan's views, such as when Melanie pleads with Halligan to surrender to Serstan, but when Halligan gives a reason why he won't, Melanie's not upset. She's also a smart lady, and probably saw Halligan's vow thing as a way to escape the situation, even if she didn't know how he would do it. And it's part of the reason why this scene, as crazy as it seems from the outside, can work well from this angle. Neither of them follows the expected rules here, and hey, it works out in the end, doesn't it?
It's also why Melanie didn't take his actions as malicious, even during the initial shock. She herself didn't want him to put himself on the line, and trusted him enough to know what to do after regardless, because she knows that Halligan probably will not just let her die. She doesn't want to be alone, either, and she wants to know he's okay. So really, what I'm saying is: There is a dynamic here that goes unspoken and that is never taken into consideration when analyzing this scene.
I realize this is pretty out there, even by MOTD standards, but it's an interesting take on this probably unintentionally strange sequence.