What you are essentially arguing is that if we cant prevent every single shooting we shouldnt do anything. Nothing is going to be perfect, we all know that. Mark Cuban quotes Churchill (I think) on shark tank quite a bit, where says "perfection is the enemy of progress". Essentially meaning, that if we only do things if they are perfect, then we are sacrificing a lot of progress that would be improvements. Things are rarely perfect, but progress is possible without perfection. . There is no way to get rid of all gun crime, just like DUI laws doesnt mean that nobody ever drinks and drives, but it does at least make it better.
Also, if you have waiting periods he also wouldnt have been able to just walk in buy any weapons. Again, obviously not perfect but if he had to wait 30 days to actually obtain the weapons it is possible he could change his mind, or maybe in that extra time someone would be suspicious and notify somebody about the things he is sending them on FB or whatever.