Part of the problem is that the universe they created doesn't really fit with the idea that Giles was wrong. Whether it was morally wrong or not.
For one thing, I have to say the idea of sparing Ben after killing members of the Knights of Byzantium to protect people feels a lot like a trope most modern fans don't have a lot of respect for, running through a ton of mooks with actions that either killed them or should have killed them and then sparing the big boss.
I don't know if Glory getting human feelings is supposed to be this sign it's safe to let her live. I think it might have just been to explain Ben getting worse. Buffy was immediately followed by Angel, and season 2's big plot was him going dark even with his soul and all the human emotions it brought and his big enemies were dangerous humans at Wolfram & Hart . Plus grief and anger are human emotions and Glory not getting to go home could easily trigger them. Plus don't forget Anya's newly human behavior in Dopplegangland.
The writers also would have known season 6 would be all about human evil, and although I'm sure it wouldn't have been in people heads yet, Jasmine at the end of Angel season 4 shows the danger of leaving a depowered bitter god around
Now, a further problem with the stance it's wrong. Buffy is likely one of the few Scoobies who would feel that.
Anya and Spike obviously would have no problem doing it. Giles did do it.
Xander brought it up, and it's unlikely the guy bringing it up is going to be totally against it. Also, the next year he was only against Willow killing Warren cause he thought
Willow would likely think she should be against it, but not feel it. And we do know what she'd do the next year
Dawn was captured of course, but she had the same opinion of Willow going after Warren as Xander. Also, I'm not sure that Buffy's morality is the standard for their family. Joyce brought a fire ax down on Spike's head when she thought he was just a person threatening Buffy and when Faith held her hostage she said she thought Buffy would KILL Faith.
Tara is the only one of the group who would be against it
Hell, let's expand it. Just recently left Riley was a soldier. He likely gets the idea of collateral damage
Oz? He's a question. Based on Wild at Heart's end he might accept it. Or he might not. Let's move him to the no category
We've seen him go dark. We've also seen him do things for Buffy's own good without her approval. You could probably sub him in for Giles and the only difference would be no bit for the glasses.
Wes? Similar upbringing and training to Giles. He'd do it.
Gunn? He'd be against it, but as we see what he does with Fred's old professor, he'd do it if necessary.
Fred? We know what she had to do to survive in Pylea, she wanted to kill her professor and the idea she needed to be protected from that was a nail in the coffin for her and Gunn's relationship
Cordelia might be a tough one. She grew to become a very caring person. On the other hand, she actually promised Angel she would kill him if necessary. I can see her trying to reason with Ben to kill himself for the good of the world, and then when he refuses doing it.
Faith would probably be traumatized by it, but I think she'd do it in a similar way to Giles and Angel, for Buffy
Lorne probably wouldn't do it, based on the way he felt at the end of Angel
So the anti-klling Ben group would basically be Buffy, Tara, Lorne and Oz.
It's hard to sympathize with Buffy's position in that case. It feels like endangering the world for Buffy to feel good about herself, at a point where all she's doing is feeling bad about herself anyway