You did have a reaction to it yourself though. I think that might be the point.

I too thought that Itta's insufficient, or rather completely lacking sadness didn't make sense, not only when you fight this boss but also when you first meet Itta's mother.

But an even worse thing happens when you first meet Itta's mother. POV: you just had your family killed, and you see your mother dying too. But she's still alive, and whoever is slowly killing her is nearby. Turns out it's some monster that no one is going to miss. You have a revolver, and you just killed to protect yourself. Also, unlike your dying mother and other inhabitants of this place, you are not the type to accept the status quo, you are trying to change it all throughout the game. First you are trying to find your mother, then your way home, then Mono. Given all this, what do you do with the murderer of your mother? Maybe you rip and tear this unholy fiend, drown it in its own black blood? Or at least shoot it until it dies. Either way, you have to kill it, there's a chance this might save your mother after all, so it's worth trying. And if there's indeed no way to save her, you will at least avenge her. You probably have some thoughts about avenging your family, especially when it has just happened and so you aren't exactly in stable and kind mood. And even if you can't kill even to protect someone (which is not true after killing that golem), then you can at least try ripping out these black veins killing your mother.

That's what would make sense, but the reality is that you do nothing. You are totally okay with this monster devouring your mother, and you are as calm as a rock, so you won't do anything impulsive, like attacking it. You won't even burst into tears or otherwise be sad.

If it was REALLY necessary to keep the game linear and prevent you from killing one of the bosses prematurely and possibly saving your mother (the latter could radically change the story), it should have at least been explained somehow. For example, if you shoot this monster, it quickly runs towards you, pushes you, and before you get up it escapes. This would also solve another not-making-sense thing about this scene. When you enter the church, your mother is behind you, this monster is in front of you. When you leave the church, both your mother and the monster are absent. The question: how did the monster leave together with your mother without you seeing it, considering you were between them? So a cutscene where the monster escapes after you enter the church would make sense whether you attack it or not.

Also, when we encounter this monster later as a boss, it's MANY TIMES bigger. Which might mean that it got so much bigger due to devouring Itta's mother, which means she was far from dead when we encounter her, and she could be saved. Also, if we can defeat this monster later, then defeating it back in church was much easier.

Now, I'm talking about things that would make sense. As I understand the game's story, even if we killed this monster, Itta's mother would die either way, it was her fate to die to make Itta more murderous and willing to change the status quo, like it was fate to die for shepherd's wife and sheep, architect's friends etc. But at least her death could make sense and really be unpreventable. The way it is it was infuriating and immersion-breaking.