It's not just that Morgan is now trying to live by the philosophy that all life is worth saving, but he's also worried that if he loses track of that ideal, he'll revert to the exact opposite, and he'll be in "kill everything" mode again.Fantastic episode. Like most I wasn't really crazy about the new Morgan either and thought his refusal to kill came across a bit forced and strange, but this episode did a brilliant job of not only making that change seem totally plausible, but actually kind of noble and beautiful. And by the end it actually made me love the Morgan character more than ever.
I think he's afraid that there's no middle ground for him. It's either save everyone, or turn into something that could be just as deadly a threat as the wolves are.