Every gun was legally obtained.
He passed every background check.
He broke no law, until he broke multiple laws.
He had no evidence of mental illness, but he was a liberal Democrat.
Just like every mass shooter before him.
Indeed, if he’d wanted a full auto license and was willing to go through the full anal exam one must to get such a license, he could have gotten one and used a MG 42 or a Bren Gun if he could have found one.