Those are interesting viewpoints, but I've always assumed he was saying this to himself. He says the words quietly. They didn't seem the right tone for a warning, more like internal reflection, imo.
He's disappointed that he isn't killing them and he's telling himself but probably also a 'never again' because his friend is gone and he could not prevent that.
I feel like it has multiple meanings.
I think the extra emotion in his intonation could be just his anger and guilt at not being able to stop it seeping into the words while not meaning anything more, but it's certainly possible. Telling people one thing outright while holding a special meaning for the words himself--it's pretty much one of his favorite things to do.
I think the most obvious choice, if he was talking to himself, would be never again would he let someone do that to a friend. But he's also messed up enough (and experienced things never going the way he wants) that I'd think he'd learn his lesson that that's not really a promise he could realistically make to himself--and he sounded serious enough for it to sound like a realistic promise.
Another option could then be never again would he allow himself to be that invested/vulnerable, but since this happens after he's indulged in a fade kiss and right before a real kiss plus I love you, it doesn't seem like that's something he wouldn't be obviously (even to himself) lying about either.
So I'm not sure either of those speculations work. What other meanings were you thinking he might have had? Or could it be less personal and more to do with his "big plans?" Never again will spirits be so vulnerable or will stupid human mages be so arrogant or powerful type of thing?