I think you speak the truth. I would say, that for some of the more notorious haters on here (eg. Fastgrass and his various aliases) it seems to be a problem with Britain in general. In their cases, I'm convinced that if Murray were not British, they wouldn't bother to single him out. Other, mostly English posters (eg.The Bawss) have never forgiven Murray for his 'anybody but England' jibe with Tim Henman when both were joking about their countries' respective chances in the forthcoming World Cup. In the third major case, it seems to be Fedfanatics and their resentment that Murray should have beaten their hero as often as he has done. In their case, I've noticed the resentment takes the form of trying to belittle him as much as possible and dismiss him as some sort of freakish non-entity whilst excusing all Federer's losses to him on the grounds of Fed's state of health, physical preparedness or, these days, his age or a combination of all 3.
Incidentally, the same Fedfanatics who take this line with Murray are generally the same ones who attack Nadal (the only other top player with a consistently positive H2H against their man) with even more venom. They don't feel they can dismiss Nadal's worth as much as they can do with Murray so, in his case, their resentment takes the form of out and out hate, character assassination etc.
It can be a sad old world on here, can't it?