Ben was killed for what he *might* do as Glory
No, Glory was a different person. Ben was collateral damage.

The Slayer problem is different. Glory was an insane, megalomaniac Hell God. Every Slayer is a human with enhanced abilities and the ability to cause a huge amount of damage if they go down the wrong path. They each have the potential to be either Darth Vader or Luke Skywalker. Is it worth the risk to hope that most of them will turn out to be Luke?