I’ll also disagree. Could be dad taught him to help defend the house as I did my girls at a younger age then this. I’ll wager odds he was bullied for years. quiet kept to himself. likely on anti-depressants...if his parents took him to a shrink.
Revenge/vengence is a powerful motivation...if that was the case.
My money says you are likely right.
If the shooter fits your profile (bullied, going to a shrink, taking antidepressants), then hell no the dad should not have let him near his guns.