People can't wrap their brains around the idea that certain people are capable of this. There was a case years ago, where two 10 year old boys kidnapped a two year old, stoned him to death and left his body on a train track.
I know people that worked in the courtrooms for the case, one kid was sobbing at what he had done, the other didn't understand what the fuss was about. He was plain and simple a psychopath.
The public and the media couldn't accept that this kid was capable of this, he must have been corrupted somehow. Eventually the media found out that the boys father own a copy of Chucky, and they latched onto this idea that somehow this film had caused it even though there was no evidence of any sort that the kid had watched the video at all.
With tragedies like this people always look for someone to blame, and who ever the target of this blame is (regardless of how involved they are) can get more hatred than the actual perpetrator.
I wouldn't be too concerned with the relatively little Mass Effect hate happening here, gaming is still gaining a reputation as a genuine form of entertainment like books or films so its an easy target. The same thing happened to films when they first became popular and widespread. Occasionally films and books still get this treatment as well, and this isn't like Brevik stating he used CoD to train for his shootings. The relationship is peripheral at best and will be forgotten about fairly soon, especially as it wasn't the person it was orginally stated to be.