People don't like flat out cheating, nor having genuinely good battles stolen away from them from a petulant child. It's also Rosberg's attitude to the fans, he cheated, he was caught, everyone knew what he did(both of the big things and many other little things) and he came out and blamed the fans, acting like he did nothing wrong.
If Rosberg had apologised first thing he said, said he let his emotions get the better of him made a mistake and he was sorry people would be much more forgiving, instead he came out attacking everyone else.
It's also the cold and calculated thing, the Monaco thing was calculated, he'd obviously thought about it earlier, what could I do if I happen to be ahead in Q3 to ruin his lap. Likewise at Spa there was the thing where he turned up angry about an insane team order that basically everyone agrees Hamilton should never have been given and was absolutely right to not slow down. Rosberg turns up weeks later still angry about that with a plan to show Hamilton his place, then knocked him out to steal a win.