I’ll presume he means that ones they CAPTURE....
Now, as to ‘what’s wrong’: it does nothing to deter the crime(s).
What we/they SHOULD do is, upon reclamation of a body (suicide/battlefield/etc.) or after execution, bury them on a pig farm after pumping their heads/guts full of pork fat.
Yeah...way too humane. You want deterence?
Bind their hands and feet, feed them alive, to starving hogs. Let women open the gates. Video the executions and post them where moslems can see it.