The man who shot up the movie theater purchased his guns legally. Is a criminal* who is dead set on killing people going to get a gun no matter what? Probably.
Is a person who is only thinking of possibly killing or robbing or whatever going to whatever they have to do to get a gun? Maybe. They may just change their mind too, if it’s too hard to get a gun, or if it adds to the amount of jail time they might do.
Regardless, making guns harder to get means they’re harder to get for everyone, and that is a good thing.
* a criminal is a person who has been convicted of a crime. The man who shot up that theater was not a criminal. He was a recent college graduate who apparently decided to shoot up a movie theater. Easy enough with current laws, not so easy if those weapons were illegal.