It's nothing to do with killer mentality. Ngannou has great killer instinct in MMA. He's afraid to gas in boxing. It was quite obvious against Fury. But I would be curious to see if he steps it up in a rematch and takes some chances with his cardio and really test Furys metal, which he didn't do in the first match.
In addition to cardio, he's probably respectful of boxers greater abilities to counter punch. It's unknown to him exactly how much better they are at that compared to UFC fighters. So why risk it. He's sparred boxers but that's controlled forms of training.
It was actually Fury who underestimated Ngannous ability to counter punch. But that was One of the things that made me believe in Francis being competitive all the way. It also stopped Furys aggressive confidence.
One thing that really surprised me was Furys lack of a jab but I don't think he ever felt settled and in rhytm to throw it like he used to.
I ultimately didn't think Ngannou would win in his debut since I don't think most of his punches are enough to Keep fury down. But I do think he punches harder than Wilder since he's bigger and still got very fast hands.