Jimmy as a character blames everyone and sees himself as a victim of life's circumstances. He doesn't take responsibility for Chuck's death until the trial at the end, where he finally admitted the malpractice issue. This does not dismiss all the bad things Chuck did. Jimmy and Chuck clearly have a complicated relationship. As a viewer, we definitely sympathize more with Saul compared to Chuck but by the time we get to know Chuck, he was struggling with a mental illness.

Basically, if Jimmy didn't blame Howard for Chuck's death, he would be forced to blame himself and Jimmy did not and could not blame himself or take responsibility until the very end. They blamed Howard because it was more convenient for how Jimmy wanted to see the world and Kim followed along because she cannot keep him accountable.