Was he "a known unstable person", or was he "that crazy guy"?
The two are different.
Many people, now, after the event, are saying "yeah, we knew he was nuts".
But few people expect someone, even if nuts, to do this kind of thing.
Was he crazy enough that people should have taken action to get him help, but didn't...
Or was he just sane enough that people discounted the possibility that he would do this kind of thing?
Are the two the same thing?
In my mind, this is NOT a result of gun control laws being too lax, but rather a result of someone not getting the proper care for their mental health.
If he had gotten the proper care, he either would be institutionalized and thus incapable of getting a weapon, or not in a state of mind to want one for this purpose in the first place.