Oh if you can do it, that means everyone can. 1.5 seconds is still a lot of time for bystanders to get in and take down the shooter.
What's stopping people from making chlorine gas in a crowded place is: 1. that's more complicated than getting a gun. Killers typically aren't smart reasonable people. 2. They'd risk killing themselves 3. It's not as directed, so they might end up killing the guy next to them, but not the woman they hate across the room. 4. Good ventilation systems.
The thing about criminals is that they aren't just evil people always breaking the law. They're people like everyone else. If you make a law saying they can't carry a gun somewhere, say a store or bar, then they have to actively break a law before they've committed another crime before murdering someone which provides another chance to stop them and another chance for them to reconsider. If it's just a random shooting, like the killer got in an argument with someone and decided to "stand his ground", then he wouldn't have a weapon if he didn't want to first break the "no-gun" law.