It's actually Evra they dislike and United by association. Gamesmanship is a big thing here, finding an edge over your opponent by any means is part and parcel and gets emphasised from a very young age (how else can we ever beat Brazilians otherwise????? ). The whole thing is predicated/viable based on a code: "What happens on the pitch stays on the pitch". Breaking that code is therefore the most frowned upon thing here, more than anything else... and that's what Evra did.
To be fair Evra had only three options:
1) Don't do anything and let Suarez racially abuse him.
2) Do a Zidane which would resulted with him being sent off and have a lengthy punishment while Suarez wouldn't be punished and would had his last laugh. Very likely the reason why Suarez racially abused him.
3) Do what he did.
I think that the third decision was the right one, though the second one would have been very good to see.