Wasn't Sentry supposed to be a dig at Superman and that's why he dies all the time?
Depends on who you ask, I think. Officially, he's a post-modern/metatextual/other fancy words take on the concept of retroactive continuity itself. He debuted at a time when readers were really starting to sit up and take notice of the industry itself and more 'fannish' terms and concepts, for good and ill.
The original deal was that he was an early Marvel character who'd been completely forgotten about and who was getting a revival. His backstory was he was a Superman-analogue that had defeated his arch-nemesis, but saved the world in such a way that he'd essentially retconned himself out of existence in every single reality, including ours (once again: 90s Marvel was weird as hell). In reality, it was the writers trying to do the kind of weird experimental story DC/Vertigo had basically perfected by the mid-90s and which Alan Moore and Grant Morrison had made a comfortable living out of for about 20 years at this point - if you've read Miracleman, Doom Patrol or Flex Mentallo, you probably know this story already.
Anyway, rather than being an interesting experiment and put back in the drawer with all the other vaguely interesting characters who never amounted to anything, he was folded into the Marvel universe as a whole. See, weird thing about Marvel, they have plenty of Superman analogues, but they've never had a
successful one. Hyperion and Gladiator are almost direct rip-offs, but they've never caught on massively. Thor's probably the closest they've got, but he's different enough that most people wouldn't say he counts. So they added him to the Avengers, gave him a big push and... basically made him look like a complete weenie in the eyes of comic fans.
Here's another thing about Marvel: in the old days, the thing that set their characters apart from others was that they had relatable problems. So Spider-Man is a big hero, but he's also permanently broke and struggles to repair his costume and webshooters and has to beg his boss for gigs. Tony Stark is a genius billionaire, but he's also an alcoholic. The X-Men have amazing powers, but they're also a dysfunctional family and the world hates them because they're weird. So, either to differentiate Sentry further or as a callback to that era, they gave him a bunch of mental problems. Great idea, plenty of people have depression, PTSD, mental illness, and in the 90s/00s, no one was talking about that. Buuuuut either they didn't know how to portray that reasonably or the writers that followed were lazy, so it was used as a crutch to get him away from fights or situations that he could solve in five minutes or less. Once or twice would be okay, but it happened repeatedly: bad guy would show up, something would happen that would set off Sentry's PTSD or fears, Sentry would fly off to Jupiter and the rest of the team would say 'oh look, Sentry's fucked off screaming into the night. Again.' At some point, it turned into a running gag and from there, into a way of basically saying 'yeah, we know this guy sucks, but Editorial says we have to use him'. Which was when they started trying to kill him off. But he'd come back. So he'd die again. And he'd come back again. The end result was that they'd written themselves into a corner. A good writer can do a good Superman story in their sleep, but guys like Brian Michael Bendis, who was writing The Avengers and was Marvel's head writer in all but name at this point, were more interested in street-level heroes like Luke Cage and Daredevil, so Sentry would get pushed offstage every chance he got. I'm not going to say Bendis was responsible for 'ruining' the character or any of that nonsense, but he was the guy in charge and the rest of the team took their lead from whoever the boss was, so...
Anyway, he may not have initially started as a shot at Superman, but a bunch of mean-spirited writers who never had any real ideas on how to use him certainly turned him into one. Eventually, Marvel came up with the Blue Marvel, another 'forgotten' Marvel character who was generally agreed to be 'the Sentry done
right' and the Sentry's in Comic Book Limbo, having been written out during the King in Black crossover. Incidentally, Bendis would eventually go on to write Superman for DC. The results were... polarizing, to say the least.