Plain, certainly, but I wouldn't say simple. While his methods were from the stand point of law, order, and civilization appalling, from a cultural perspective he is a hero. Don't misunderstand, I'm not defending or justify what he did, but look at it for a moment from the perspective of the common narrative motif.
If this story were to be pitched in Hollywood, would Dorner be the hero or the villain of the movie? Distinguished military veteran and policy officer stands up to abuse in his department has his life ruined by said department wages a one man war to bring down a corrupt and abusive government authority that maintains order with an iron fist and a shoot first and ask questions later methodology. Were this Hollywood, Dorner would not only have evaded capture but would have exposed the corrupt mayor/police chief, brought down the police department, and cleared his name.
Every rogue cop movie, every story of a rebellion vs. an empire (and honestly, where are you going to find something closer to a real life Imperial Storm Trooper in America than the LAPD?), every story about a man standing up to an unjust authority tells us he should be the protagonist of this story. Is it any wonder so many want to think of him as one?
This sums it up pretty perfectly. Captures exactly why the fan club caught on and why so many were cheering for him.
It reminds me of the movie Law Abiding Citizen.
Who at the end of the movie wasn't pissed off he got his bomb discovered and was rooting for him the whole movie! Very few people I would bet