I understand what you’re saying, that any sensible lawyer would keep previous convictions from coming into play here. The rap sheet plays a huge part in the punishment phase.
I don’t see anything here which would be self defense, he purposefully confronted and attacked the man after he left, then fled the scene. I don’t think a jury would nullify under those circumstances.
And finally found some of his California cases; he probably would have been looking at a guideline range of 12 to 25 if the jury went light and found him guilty of manslaughter.
From what I can see, it is 4-10 years for manslaughter and 10-22 years for 2nd degree manslaughter in Arizona. Presumably he was charged for 2nd degree murder and plead down. Thinking about it I agree that he should have gone to trial. Even though he was probably guilty legally, there would be a high probability of acquittal, a huge jury, or a manslaughter conviction, because of how some members of a jury would react.