There are historical reasons as well as footballing why Liverpool is to be hated. Doesn't mean you have to but try telling Gary Neville you'd prefer Liverpool over Chelsea. Wouldn't go down well. If it's between Arsenal and Liverpool as well, Arsenal gets the nod IMO. Just to be in good graces with United fans who know the history of the club and what it's about don't go showing any love for Liverpool.
I know loads of United fans (obviously, I suppose we all do) and I post regularly at United forums. There was even a discussion on this very subject and people agreed it doesn't matter if you do or don't hate Liverpool. I'm in their good graces well enough.
Something tells me Gary Neville wouldn't give a second thought as to which teams I preferred after United, let alone that I supported United in the first place. He doesn't even know who I am.
Maybe you should ask Ronaldo if he hates Liverpool. Doubt he gives a crap either way, either. Maybe he'd support Chelsea on the "Portuguese compatriot" basis.
After looking at the thread about who "supported" which team in the Chelsea vs. Liverpool semi in the CL last season, 5 said neither, 3 said Chelsea and 13 said Liverpool. And there you go!
Think of it like having a rival for a long time and then some bastard upstart comes along and tries to outdo the both of you. Haven't you ever seen it in comic books where the good guys and the bad guys team up to beat up the "new big bad guy"? It's like that, I suppose.
But more to the point for myself, you just
can't "force" a hatred. If you hate something without any reason but "everyone else does" then you're clearly incapable of thinking for yourself. You can want to beat them due to things you read about with regards to the past, but you can't hate them just like that. Especially when you don't know any wanker Liverpool fans. Maybe you just know a bunch of wankers that happen to be 'pool fans, Rose?