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Tear Jerker: Hawkeye
In his first limited series, it turns out that his girlfriend has been manipulating him all along. The worst part is the way she tells him after having him thrown into a pit which they then begin to fill with industrial waste to try and kill him and Mockingbird.
Sheila: Yes. They paid me. I was pretty convincing, wasn't I? You never had the slightest idea that I could sooner love a dog than a cornball Romeo with delusions of adequacy like you.
The entire story itself is essentially everything Clint fears about himself: he's not good enough at his job (he doesn't realise that he's being tricked), he'll never find a woman who loves him (his girlfriend turned out to be working for the villains), and he almost kills Mockingbird near the end of the story, thanks to Crossfire's tech. Thankfully, at the end he wins, Mockingbird isn't dead, and they even get married.
When his wife, Mockingbird, died in his arms at the end of West Coast Avengers #100. He was so heartbroken that he spent a long time in the middle of nowhere and only went back when one of his old mentors told him that Bobbi wouldn't have wanted him to live that way. Sure, the Mockingbird who died turned out to be Skrull, but that doesn't make the panel any less painful.
House of M is sad enough, but it gets even worse if you've read House of M: Avengers and see Clint in a relationship with Bobbi before she leaves for Wakanda. This means that when he gets his memories back, not only does he discover that he's dead in the real world, he discovers that the woman he loves has been dead for years.
Even more heart-wrenching now. Clint and Bobbi being together is their ultimate happy ending, was that even when she was dead, and now it looks like they won't even get that in a universe where they're both alive at the same time.
At some point when they were West Coast Avengers, Bobbi got pregnant, and she and Clint were really looking forward to having a kid, to the point where they worked out when the kid would have been born.
Bobbi: Before all of this, when we were Avengers, a couple of years ago... I-I had a miscarriage. We didn't tell anyone. Me and my husband, we figured his or her birthday would have been October 12th. We really wanted that kid. And October 12th... That... would have been a nice day.
That panel in Hawkeye & Mockingbird where Clint and Bobbi realise that, due to their current PTSD and adjustment problems, neither of them are in a place where they can be in a relationship with the other. After everything they've gone through, all the times they've worked on their relationship, seeing it end there is awful.
Even sadder, the series was intended to be an ongoing that got cancelled despite positive reception, and judging from Widowmaker, which was intended to be an arc but used as a plot thread tier-uper, looked like they still had strong feelings for one another; its likely that, had it continued, they'd move past their issues and get back together eventually. Never has an Aborted Arc been so sad.
The ending of Hawkeye 9. It's brutal and not something you see coming.
And in a Kick the Dog moment for the reader, it replays the ending of Issue 9 for Issue 10.
Even before the ending, Hawkeye #9 delivers a gut punch with Clint's defeated little comment when Kate is trying to convince him that he isn't a complete screw-up.
Clint: Look at me. Look at all these things I've done.
Even more upsetting is the implication that he isn't just talking about messing up his relationship with Jessica, but his relationship with Bobbi as well, and bringing his friends and neighbours into a mess that had led to him guarding the building.
Secret Avengers Vol 2 # 15-16, first he finds Bobbi, who's mind is currently breaking thanks to what Forsden did to her memory, not only have no memory of either herself or him, but then finds the idea that they were married funny, which is visibly traumatic for him. Then once they've nearly got her out of there, he watches her seemingly get shot and bleed out in his arms, only to turn out to be Yelena, with Bobbi having slipped the camo tech onto her while they were fighting. In the end, his ex wife is now out there, missing, with him having no idea if she's alive or dead, and if she is alive (which she is, thankfully) he has no idea if she even remembers him or if they could ever find her. While she's thankfully safe and with Bucky and Daisy, the whole experience is enough to make him quit SHIELD in disgust. Its even more painful when you remember that Clint and Bobbi didn't break up because they didn't love each other (and even after their breakup, its made clear a lot that they DO still love each other a lot), but because of other issues, and that, because of the memory tech, he won't even know what happened while he's not working with SHIELD.