I think you're thinking a bit too deeply into it. Her redemption arc was that she helped save her entire people from the Goddess of Death. Not really 'helping this guy out for reasons'. Tony Stark was an arms dealer ffs and he's beloved and the face of the MCU. Hawkeye went on a full on murder rampage, Antman and all of the GotG are criminals. God knows the Atrocities that Gamora and Nebula committed for Thanos that no-one cares about. Their redemption arcs were even less 'for reasons.' Seems like a bit of a nitpicky thing to have such an issue with. Scarlet Witch enslaved an entire town inside their own minds to the point where it was driving them crazy and Loki tried to subjugate all of Earth and no-one has an issue. "He killed 80 people in 2 days" "Err, he’s adopted?" and laughs all round.
So you agree its a running problem?
Tony Stark was an arms dealer, but not the illegal kind. Sure, thats probably splitting hairs, but its a hair thats need split.
Hawkeye murdered criminals, theres an argument to be made that killing bad guys is kinda what they all do, no?
Antman and GotG are thieves, and they arent being given the power over an entire race of people.
Gamora and Nebula, probably mass murderers. But again, no one is giving them power over an entire race. No one is holding them up to be worthy as a ruler.
Scarlet Witch should be in a jail cell at the very least. I have MASSIVE issues with the "They'll never know what you gave up" line at the end of that show.
Loki, again, no one is giving him the crown of Asgard. Giving it to him would be just as problematic as it is giving it to Valkyrie. Imagine how ridiculous it would be seeing loki giving a morality speech to his people with a straight face. Imagine Scarlet Witch giving a lecture about how its bad to enslave people. Imagine Antman or the guardians giving a speech about respecting other peoples property.
Theres also the issue that while all of these characters have highly questionable backgrounds. None of them have hundreds of years of doing those things, unlike Valkyrie. Who may in fact have been enslaving people for many thousands of years. Perhaps even hundreds of thousands of years. When Thor and Loki arrived on Sakaar, they exited the bifrost separated by a few seconds yet arrived separated by a couple of weeks. If we assume earth/Asgard time moves at a rate of say 10 secs = 1 week, its pretty easy to assume that Valkyrie has been there for so long that she might be the oldest living being we have met so far in the MCU. In any rate, Hela defeated the Valkyries more than 2000 years before she was released by Odins death. Valkyrie exiled herself on Sakaar at that time. So at a minium she has been enslaving people for 2000 years. 2000 years of being a dick seems like it would need more than one or two bit parts in movies to get that redemption arc to come good. No?