What gun laws could even stop this? Or like any mass shooting for that matter? Laws aren't magical binding principles carved into the fabric of reality, they're fuzzy little guidelines that the system may or may not follow at its whim or the whim of whatever enforcer is closest.
Like mass shooters are almost exclusively middle class white men with reactionary beliefs that cops sympathize with, and exactly the demographic that gun laws never target. And then you've got the ones who already can't legally own firearms or shouldn't legally own firearms but who get to own them anyways because domestic violence isn't taken seriously by cops, or because they just steal them from family members.
Here the gun in question was stolen from a family member, who was a cop, and while cops absolutely should be barred from owning or carrying firearms they're definitely the last group liberals would ever try to disarm.