Emet Selch is a deeply depressed man who'd lost his whole civilization and over the millennia since, cursed himself with the burden of always remembering, of tacking every agony onto his soul because he's afraid that if he falters for even a moment, everything will come crashing down. He creates a facade of himself so none ever see the burdens he carries, partly out of necessity because he could never explain it to the sundered people he frequently lived with and partly out of pride.

He says that he doesn't consider us human, ergo we're free reign to kill. But he doesn't kill us. He has every reason to and every ability to; he even goes sk far as to resurrect Y'shtola, and that was before he began putting the pieces together on who we were. Heck, he's delighted at the prospect that we might be able to handle the Lightwardens; it would mean he's that much closer to being done rejoining and that much closer to being done with the blood on his hands and millennia of scars on his heart.

He has had families with the sundered, led them, lived with them, and surely has their tragedies engraved on his soul too no matter how much he would strive to not admit it. So he has to believe that they're not truly alive, or at the very least, that through their death they will be made more whole, because not even he would be able to bear the burden of all the deaths he's witnessed and been the cause of. Nobody could.

Of course, that all assumes he even did mean it when he said it. Recall his tone; "Yes, yes. Moral relativism and all that. I don't believe you're truly alive, ergo its not murder if I kill you." (Paraphrase) It's dismissive, not even really being presented as an argument but more of "I really don't care to get into the weeds on this topic with you lot." I personally like to believe the former; the tragic tale of an immortal whose heart is burdened with memory, fighting always in the direction he thinks is right and hoping beyond hope that all the death means something and trying to find the strength just to wake up every day.