I think some people are giving Nemmy a bit of a pat on the back in terms of it's intellect. Don't forget that it's not exactly the sharpest tool in the shed. It had to come with pre-programmed parameters to it's main mission, as well as having an N-type parasite injected within it, in order to make the Nemesis what it is.
Nemmy isn't a moustache twirling villain, fully aware of being evil and the like. Looking at this remake and the OG game, just look at how it still repeats the same tendencies, the same behaviours, even when it's evolved to it's second and final form. The Tyrant still acts like a rabid animal, just running on subroutine programming.
Also, yes, he could have easily killed Jill, even at the start, even at the rooftop scene, where he grabs her by the head (could have easily crushed her skull like Mr X did in RE2 to Ben). The thing is, imagine you playing a main protag, and then dying in the first boss fight, and the game officially ending there, it'd be out of left field and atrocious as hell, not to mention it wouldn't make sense, and hell, movies hardly do this, because it doesn't make any sense (it does however near the end of a movie, to set up closure or a sequel). There's a valid reason as to why villains are dragged out, rather than designed around irl evil people who could easily kill someone in an instant.Using irl evil writing in a game just cuts any game short to about 30 mins to an hour at best, which makes for both shitty writing and a shitty game.