Anderson didn't really like insulting Sherlock Holmes, he didn't like having to hate the man and act how he did when around certain people. He didn't like that he had to shave off his beloved beard. But he did all these things for a reason that was probably so sad and so stupid that even John could agree with him.
He was a dick to Sherlock Holmes because he didn't want to be bullied. He didn't like being called 'different' and 'annoying', because he knew he was in some way, but just because it was true it didn't mean he had to like it.
You wouldn't believe it but Anderson wasn't as dim-witted as he was seen to be. He could see little details and certain things just like out favourite detective could and that was why he was bullied. But Sherlock didn't take words to heart like Anderson because he was a sociopath, everyone knew that.
He had to act like everyone else to fit it. Donovan was the first one to pick up on his strange way of thinking and that is why he hung around her so much. She was like a ring leader at a high school bullying circle, finding the most abnormal person and berating them for their differences, so he stuck with her.
He changed how he talked, looked and what he said just so she wouldn't embarrass him.
That's what he thought was stupid and sad. He was a grown man that would change his whole life just to not be picked on. He guessed old habits died hard. He was always the smart kid at school, the first one to raise his hand. He always loved science and chemistry, that's why he chose his job in forensics. Of cause, not going along with the fads and having no group of his own resulted in him being some what of a target for older kids. He didn't like being weak but hat was how he was.
So when Sherlock was reviled dead and over the in the next two years that followed he noticed over seas mysteries being solved in a 'Sherlocky' way, he quickly pieced together it could be no one other than the quirky Detective. In a way Anderson looked up to Holmes, the man could look after himself and always solved the cases no matter what anyone else thought.
He wished he could be as brave as to voice his own thoughts but he stayed behind the mask of another man, over the years finding it easier to be this alter ego he had created. He forgot how he had once looked up to the detective helping them out and grew an actual dislike for him. Once Sherlock had 'died' however, his normal feelings returned, the research he dug up over the years got him fired and he finally grew out of his 'protective shell', sticking up to Donovan and growing out his beard again. He loved having a beard he really did.
He stayed up at night looking over evidence and maps, figuring out when and where Sherlock had been and when he was coming back. Because Sherlock was alive! He knew he was! Why couldn't anybody see the signs? His old fascination had returned and he didn't know why he had let himself become such a person he was not.
It wouldn't happen again, he assured himself.
He wouldn't hide behind a fake personality anymore and he would convince people that Sherlock was alive even if he lost everything because of it.