What could be the cause of death for a red ear slider turtle?
I wouldn’t automatically assume that it starved at all… you didn’t mention how long it was between it stopping feeding and dying, but it takes longer than you think for turtles (even babies) to starve to death. Without having a necropsy done, there’s no way of knowing for sure… while it ~could~ have been malnutrition (pellets should only be at most about a quarter of total diet), again, it would take quite a while for symptoms of malnutrition to begin showing… You don’t mention what temperature range your tank has… if they can’t reach their optimum body temperature they can’t properly digest their food, and food rotting in their gut will kill them a lot faster than starvation… parasites, disease, there are many options on what killed your turtle, but it wouldn’t have been your brother restraining it while the smaller one ate.