Having worked at a local shooting range when I was younger, I can attest that a large percentage of folks can't shoot this well off of a bench, at a paper target. Much less at a human target.
As you well know, many people who visit a shooting range don't have a clue what they are doing. Clearly our shooter does.
Many thousands of shooters (varmit hunters, for example) are competent to manage the shots involved in these incidents.
A mentally ill person would have some lucid moments, but these crimes seem very calculated and I doubt that a seriously delusional type could keep it together long enough to pull this off, for this long of a time period. The shooter also has not lost his cool and begun to blast away at more than one target when it was probably highly possible. Anger would lead to mistakes IMO, this shooter doesn't seem angry, rather, he is careful. Also, a mentally ill person may have reason to kill a few people, but most people do have a conscience. The killer seems to have totally supressed his, giving me the impression that he was trained to do so.
I also think that while it may be possible to learn to shoot accurately, it is totally different when put in a high stress situation, especially with the possibility of the shooter being captured. He has to hit his target and keep his cool and get out of there without being seen or caught.
These reasons are why I think that a team is responsible for the shootings, and that they were trained to do this exact job.