Those are really interesting questions!
SPOILERS!!!
IT's name and species are never meant to be known -- I wanted to use soft worldbuilding to give IT a kind of mystery that makes its actions feel more alien and terrifying. IT is somehow able to bend reality and consume the wills of its victims to become stronger -- but there's no telling why IT does this. Is it by choice or necessity? Either way, the outcome would be the same for the protagonist which is why the MC has to make the choice to willingly become another victim or break free by defeating IT.
IT does find pleasure in messing with the protagonist -- like a cat playing with its prey before eating it. But again, since IT's motives and nature are so alien, it's meant to be impossible to tell if it genuinely enjoys the torture due to some innate instinct, or if that's just how IT shows affection or if it's just a jerk. IT's victims become food that give IT more power, but there's more to it than just that.
IT truly loves every single one of the victims IT possesses.
But IT doesn't know how to love humans.
It's not a romantic or even friendly love. It's a love that doesn't quit or let up -- a love that stifles and suffocates and obsesses and consumes entirely. IT's love can appear reciprocal at times (like in ending 22 or 27) but everything given is meant to be of benefit to itself in the long run. It's a love that is willing to give everything so long as it gets that and more in return. That's the thought I had when writing the story.
IT's love is meant to be as alien as IT's actions-- completely incompatible with humans, anyone would be crushed under the weight of that kind of love -- but it's also meant to be a little familiar too. The manipulation, the half-truths, lies, and deflection, the gaslighting and guilt-tripping, the punishments for not meeting its demands and standards -- they're things humans are familiar with, but we don't tend to apply them to any kind of healthy love.
I've seen a lot of people say that they'd want an ending where the protagonist stays with the cat, but that ending already kind of exists -- Ending 37: FOREVER. IT wants to possess the MC so wholly and completely that they lose their self and become part of IT. Nothing less than that will do.
Even in the end, IT saying "i hate you" is more than just lashing out upon being defeated and rejected -- it's one last act of manipulation aimed to hurt and guilt the MC for denying IT's love. With its dying breath, IT tries to possess the MC in any way it can.
Sorry for the long reply, but it's fun talking about the thoughts I had when writing the game!
Thank you so much for playing, I'm glad you enjoyed the game and the story!