“Instinct” is just a fancy word for “drive” or “desire”. The latter are used for humans, but because throughout most of history we have considered ourselves more sacred and important than other animals, we needed another word when we started studying their behavior. Sure, there are subtle differences between the ways these words are used. For example, I may say I desire a burger but I would never say I have the instinct to eat a burger. Clearly we couldn’t have evolved the specific instinct to eat burgers. But the desire for burgers is nothing but a specific manifestation of our generic instinct to gorge in fat, high-calorie food.
So yes, not only is there “something instinctual going on there”, every single time an animal does something for pleasure, it is a manifestation of their instincts. With no exception. And similarly, every time an animal does something “instinctively”, we can be confident in assuming that that instinct manifests itself as a desire, the fulfillment of which is most likely experienced as some form of pleasure or relief from discomfort.
There are many ways a mother can experience the killing of their weak offspring: maybe she just finds them unbearably annoying, maybe she feels terrible but after a complicated rational calculation she reaches the conclusion that in order to maximize her well-being and that of her other babies, this painful sacrifice must be made (“delayed gratification”, or “expectation of pleasure”, as I mentioned), or maybe they just look delicious and irresistible. But we can be confident that, unless her decision was rational, the experience must be at least remotely analogous to pleasure. Otherwise, she just wouldn’t do it. Think about it. Is there anything that you do “out of instinct”, but that doesn’t bring you any pleasure or relief from discomfort at all? Or at least the expectation of it? “Instinct” is a technical term that focuses on behavior and avoids phenomenological questions of subjective experience. “Desire” is the subjective experience of an instinct manifesting itself. If it is always like this for us humans, why would it not be the same for other animals?