What law would strop that very same person from getting a gun? Remember they just snapped, allowing them to get guns in the first place. Waiting 2, or 3 or 4 days? how would that have stopped this guy? it wouldn't have!
Let's assume for a second a country where there is a single database of gun permits/ownership, where it takes longer than a few hours or a few days to clear the application process, and where you get flagged for such things as past mental illness, misdemeanors, etc.
Now let's assume that Person X has had a mental breakdown and wants to kill himself and others with him. He has a history of seeing a counselor at younger ages, and has been a productive member of society but takes pills for depression.
He immediately needs to buy a gun. He goes to Scheels. Well, they can't sell it to him without a permit. So then he has to go apply for one. In the process of applying, the database shows his non-criminal yet still flagged history so he has to do a psych eval or whatever.
Maybe in that process he is denied the permit and never gets access to a very efficient tool for killing. Or, maybe he still does get the permit. Even then, it's likely taken him at least 2 weeks, in which time he might come out of depressive rage, someone might see a warning sign that he's suffering from some mental ailment, he might kill himself by some other means (psychotic people aren't necessarily known for their patience), etc.
Simple, non-oppressive restrictions and safeguards make for an environment where there are better chances for this story to turn a different direction. They don't stop it outright, but they give him and the people he wants to target more of a fighting chance.
Now you tell me, what are the cons of laws like this? They obviously don't protect everyone from everything, but you can't say that they wouldn't stop at least ONE person in our country from killing a bunch of others. You can't say that at least ONE of the 23 toddlers that have gotten their hands on guns this year probably wouldn't have been able to were regulations like this in place. So what's the downside that doesn't make saving even a few lives worth it?