The reason for the hate has to do with the perception of whether the character earned his bad ass status.
Let's use Ainz from Overlord for example, he devoted a large chunk of his life to playing the game that became real. He started off weak, got some help and made some friends, and earned the trust of his guild, which paid off when he was nominated to become the leader of a new one. His power comes from a lengthy time dedicated to leveling, learning the game, spending a ton of money, earning loyalty through his devotion to his guild, and continual clashes with others due to being in a pvp guild. He also knows how to take on fights in cooperation with others. His harem (and guild mates) love him because he shows again and again that he'll put those close to him first, he's earned social skills through leadership, real world work, and spending time with his guildmates, and he's a bad ass who walked the road to get there. Basically by the time we see Ainz kicking ass, he's a well-rounded guy (for a online gamer), we expect an old (relative to Kirito) veteran to be good.
Based on my understanding of what others have said (I haven't watched SAO), Kirito, in contrast, is literally a teenage kid who's just better than everyone else because of "training" (which others should have been able to do too, but somehow he gets more benefits then most.) He beats veterans and overcomes difficult situations because he's just THAT good. His harem loves him despite realistically a kid who spent so much time in game leveling, and being a teenager on top of that, would have poor social skills, until he had a chance to develop them as he aged. The justification for his bad ass status comes across as flimsy and premature.
Both of these are MMO wish fulfillment characters, but if I asked who here would prefer Ainz to Kirito, for most it would be no contest. It's worth noting, most would consider Ainz a VILLIAN.
Oh, they answered me ... Well, well.
For starters, the comparison of Ainz and Kirito is extremely inappropriate, there is nothing similar at all (we do not consider alicization).
Further, Ainz is far from being such a "hard worker" as you described here, he has so many buns, freebies and pianos in the bushes, and even without bushes he has been and will be. Oh yeah, don't forget that he came into the world along with the whole guild, a bunch of strong NPCs, a bunch of powerful junk in the treasury from other top players, with development a cut above the average in the world, that is, he is, in fact and by default, stronger most of that world Originally! And you're going to say something else about "labor"?) Don't make my slippers laugh. I also laughed at "Diplomacy", where is "diplomacy"?) There is some attempt at a political game, with a bunch of advisers, but it is Ainz himself who is completely bottomed in politics, and most successes are stupid strength and luck. At the same time, I'm not saying that "Overlord" is a bad work or something like that, on the contrary, it's a completely normal title for its genre and audience.
As for Kirito, of course, he also has a lot of pianos, which was noted by me in the first post, but some of his capabilities are fully explained. From a young age, he was a kind of "genius" in technology and programming (even in reality this is possible, though only a few, if you are stupid, then I'm sorry, you were not included in that list of 0.05%), plus, Kirito actually scored on the world and plunged into study, games, programming, on which later, in the course of the plot, he traveled. Plus, he has some skill in thinking. At the expense of the "harem", there the main person was immediately indicated plus or minus, and the fact that Kawahiro wrote each story with a new girl is the problem of the author - Reki, and what does the character have to do with it? Plus, they are more friends, not a "harem", yes, some "love" him, but there is more of a slight falling in love out of gratitude for the fact that he morally or physically helped, against the background of the fact that everything was bad with them before. By the way, a moment of humor, he (the author) Kirito almost made a gender bender, but later changed his mind and wrote that the avatar was just "feminine". In general, one has both pluses and minuses, and the second, even though this is not a correct comparison.