Not wanting your competitor to win is OK, and I can also understand old rivalry. But there's a lot of people in here willing to sacrifice their own team's success for our failure. This thing + general language used does not feel right to me.
The language used is fine .. it's a fierce football rivalry.
I don't get it, it's not like people are extending it to death threats or violence, hating your rival teams and revelling in their failures is one of the best parts of football. Is it irrational and silly? A bit yeah, but again that's all part of the fun. What doesn't feel right to you? Football is tribal and silly, you're in the wrong sport if you're expecting pats on the back from your rival supporters.
It's not like top 4 would be some huge success for United anyway. I'm sure if it was a case of 'we win cl and liverpool win the league' 99% of their fanbase would accept that, but for United as a club merely getting top four is no big deal whatsoever historically, whereas Liverpool winning another league would be HUGE. And again, rivalries mean something and how well your rival is doing is absolutely very important, it means something.
End of the day United finish 5th and what? The sky falls in? They're a mega club with stupid resources who can still sign very good players without CL football. Liverpool win the league and they're subjected to the media wankfest for months, any Liverpool supporting friends they might have being totally unbearably smug, and the very real prospect that Liverpool might be getting back on that perch.
I don't think it should even be a remotely hard choice for United fans. The idea of watching Henderson lift the league title should make any of their fans come out in a cold sweat, it's horrible enough for me and Liverpool aren't even our rivals. Missing out on CL football (let's face it, they're not going to win the thing next season) for one year is hardly a massive sacrifice to stop that horrendous nightmare.