We don't punish people based on what they "may" do but what they actually do. Neither Catholic moral teaching nor U.S. law accepts the premise of preemptive punishment, and certainly not preemptive capital punishment.

Besides, violent crime inside prisons isn't done just by convicted murderers but by a variety of offenders. Do you think we should execute them all?

What you're talking about is a matter of prison reform, which I'm entirely for. But it makes no sense to say that we should execute people because prisons aren't safe. That doesn't fix the problem, it just compounds it.