A: Since your son was a very gentle person before, you already taught him proper behavior by raising him lovingly. But you are right that the cruelty your son is showing to the kittens is a sudden red flag. It sounds as though he is overwhelmed with aggression that he cannot understand, which he is taking out on the kittens.
If there are problems at home related to his live-in grandparents who are suffering from Alzheimer's, it could certainly add to his level of stress and confusion. But the fact that your son is bullied at school could be the main source of helpless rage. Bullying can lead to long-term damage of a child's personality, so take prompt action. Meet with his teacher to discuss what the school can do to stop the bullying. See if your son returns to his usual self once everything returns to normal. If he doesn't return to his usual self, ask your family doctor or pediatrician for a referral to a mental health professional who understands children and families.
Avoid punishment, which will only add to his overloaded anger. Instead, offer empathy and supervision whenever you can. Show kindness and respect to put him back on the right path; he needs your support to continue reflecting good character qualities.