Think I said it before, his sister looks to be like the controller to his power (There's also that art of her on his back and he's kind of like a mount, allegory I guess), which could mean she either is real through some weird stuff, maybe the alien hivemind is messing with his head (and the aliens are sort of "controlling" him, unlikely), she's not real and he's just delusional, which means she's either going to be quite static in her opinion (maybe his sense of karma/justice?), or, she's a changing delusion in which John could regress purely by deluding her to being more active/ruthless, in his first transformation John showed regret for having hurt innocents, but I don't think we ever saw whether Emma regret anything, it's unknown her personality (though we know she blew up that camp, whether there were innocents there too, probably I'd say)
She wants the best for John, so if John doesn't want to kill someone but they're going to kill him, I reckon Emma will kill them, purely out of John's control
It's cool, I'm not sure I've seen this done before, and there's a few ways it can be taken, John could even stop seeing delusions of her in a way to "master" his transformation (he moves on from his grief, a "stop living in the past" moment), kinda feel like that's the endgame, he stops relying on her, as she's been trying to get him to do, he was so reliant he started having delusions of her, crazy
Second time his sister transformed him, first time he seemed sort of mindless, this time not, but he's also not fully transformed, maybe it's visually him moving on from his sister, pretty cool