Going down the darker route.. I like it.
Maybe he returns to the Zoldyck estate a little after Killua and Alluka just left and finds out he just missed the chance to see his big brother again, and not only that, he’s looking after and caring for his older sister. After all this time looking for him, hoping he’d come home, Killua just blazes in and out, not even asking about poor Kalluto.
Also, maybe like Killua, Kalluto always recognised Alluka as his sibling, and like you said, he looked up to her, though he stayed quiet about it once she was sealed away.He always liked her, but now she’s stealing his brother away, and because of her he might never come home again and that just can’t happen.