Please explain...

It started as his discovery towards the impact he could have on a life form, realizing he has this power to utilize.
This then lead to being put into force when he didnt like the manner in which he was being treated by his parents, and the fact his father had intruded on his place within the house (i.e - the relationship with his mother, which yes was perverse but still the life he had grown up to believe and experience)

it's also important to note that children (and Bubby is basically a child) who are systematically abused by their parents or authority figures will go on to abuse animals or younger children whom they have power over. as well, Bubby hasn't learned any other way of treating someone. he doesn't know he's torturing the cat, so of course he doesn't know it's wrong. this falls more on Bubby's mother's shoulders than on Bubby's.

Bubby treats the kitten he finds later differently. he handles him gently. he calls him 'good cat'. he starts to speak to him in his mother's aggressive voice and then stops himself, and you can see a look of realization on his face that this is wrong. he goes to get pizza for the kitten in an attempt to provide for and take care of him. he is genuinely upset when the kitten is killed.

i feel the importance of the scenes is to indicate how Bubby is learning and growing and changing in the outside world. he is realizing that what he was taught and the way he was treated, and in turn the way he treated the cat, was wrong. and as for using animal abuse as a plot device, in a closed-off world like the one Bubby was raised in, i can't think of another equally-probable vehicle for Bubby to have displayed his power over. he would not have treated his mother in the same way he treated the cat, and there was no other living thing in the room.

i can only speak for myself, but i didn't appreciate the scenes because of the acts of animal cruelty displayed. i appreciated the scenes for what they said about Bubby and his growth into a loving, compassionate person who could develop the ability to care for someone.

Bubby did went on to commit violence against humans. He killed at least 4 that we know of. The cat in the begining was crucial to display bubby not trusting his mother and experimenting with whether what she told him was true or not as well as replicating his mothers actions to others. The second cat was the moment when bubby understood what death actually is. Both of these cats were not just pointless shock addictions but actually critical for the plot, which makes them good additions.

Untalented cruel idiot has money and makes movie, simple.