You think so given Heysel, after which the English teams banned from Europe for several years, and the recent racism saga and how poorly Liverpool handled it? United have not done anything like that to invite hatred.
It's not necessarily rational hatred. You are hated more due to how successful/dominant you have been in recent times and as a result people develop negative feelings towards you. Also there was the whole thing going around about how you had the refs in your pocket/conspiracy theories which stoked up hatred.
People enjoyed your fall immensely, that's for sure. Reveled in it in fact. Although the same would probably be true of any team that had enjoyed so much dominance for so long, but like I said that's what has contributed to the dislike people have for United. There's also an aura of arrogance around the club and some of the fans.
Racism saga had impact for sure, but I'm not sure it propelled them to most hated. I certainly don't think they are a 'loved' club or whatever, I just don't think they're THE most hated or that everybody dislikes them. Most people I know wanted them to win the league. Heysel was a while ago and not sure it has much say in whether or not they are hated anymore for the majority.
I don't personally hate either clubs and am quite fond of United but overall most opposition fans don't much like you. Remember Sunderland fans were delighted after you lost the title and celebrated it? They're a bunch of cnuts and might have done it to anyone, but still.