He didn't have a backup gun.
The next question in your line is "well how were the cops supposed to know that?"
The answer is "Oh so every cop can just shoot anybody they try to arrest because they MIGHT have a gun hidden on them somewhere? No, they cannot. That's not how the law works."
Your next reply will be "Of course not, you can't just assume everybody has as gun, but Alex had a gun removed from him so it's reasonable to assume he'd have another one."
My answer would be "No, it's not. Having one holstered gun doesn't guarantee you'll have another one. See my first theoretical response. A cop can't just shoot somebody because they think that person MIGHT have a gun on them."
Come on, it's like you're not even trying.