The question is not if he should have been arrested, but why he was allowed to obtain guns.
The sole purpose of background checks is to prevent possibly/certainly dangerous people from obtaining guns.
He was crazy.
He was a wife-beater.
Either/both of those two should be sufficient to bar his purchases.
The FBI could have requested the gun dealer not to sell him a gun, even if they were legally unable to block a sale.
There was NO record of his issues for a gun seller to reference, he had no charge or police report/record of domestic violence, she didn’t report it, no psychiatric history officially, please, people, get up to speed before spouting off
With neither a criminal conviction nor a medical opinion that he's dangerous? Just some bureaucrat's or LEO's. I'd suggest your trust in government employees is misplaced.