OK, so some murders do reoffend. That raises two issues.
Should we execute people if there is a reasonable probability if them killing someone in their future? Is the only indicator of them murdering someone in the future the fact that they have murdered in the past? Are they better indicators and, following your logic, we should then execute these people. What is the cut-off proability of a future murder at which we decide that someone should be executed?
Secondly, some people are unjustly executed. How many future murders does one have to prevent to justify each unjust execution?