Oh boy. Most of the reasons for hating Shadow seem to be a little off the mark, in my opinion.
First and foremost, hating him for being “emo”? I’d say he has a reason for it, given he (along with everyone on the Space Colony Ark) was betrayed by the Military, and his only friend was killed as a consequence. Plus, given the time when Sonic Adventure 2 had come out, “brooding emo” characters were the trend; it was just good marketing, when you consider how well that Shadow was received in SA2.
Which brings up an interesting point; Shadow was intended to die in SA2, and that was supposed to be the end of it. However, because he was so well-received, Sega ended up “bringing him back” for Sonic Heroes and everything thereafter, including his titular game. But just because Shadow the Hedgehog was overall poorly received is not a reason to hate the character; that’s just misplacing hate that would otherwise be aimed at the game designers.
Overall, he was actually one of the more dynamic characters in the Sonic universe; I’d go so far as to say he was one of only two characters out of the entire Sonic Cast who was not blatantly one dimensional (the other being E-102, but that’s another matter altogether).
He actually shows character development throughout the series, especially in Sonic Adventure 2. Even in the poorly received Shadow the Hedgehog, he at least develops in the story, which is something that can’t be said for many other Sonic characters, who are generally the same throughout the whole game.
Overall, he may have suffered from a forced rebirth and a little bit of bad writing, but that’s hardly the fault of the character. He’s actually one of the most well received characters in the Sonic fandom; for a “Sonic Clone”, he’s a hell of a lot deeper than Sonic himself.