Exact.What if Trunks only killed him because he physically split him? Zamasu seemed immortal to penetration, ki blasts, and physical attacks, but getting sliced in half? It was never made clear if he had Boo-like regeneration for a situation like that. It could be an explanation to keep Trunks below Vegetto and keep the writing more consistent.
Interesting theory but DB doesn't really work like that. For Trunks sword to penetrate Zamasu in the first place means he had to have had comparable power. We saw in the Android arc how Trunks sword couldn't slice 18 when he tried to cut her in half because of the power gap.What if Trunks only killed him because he physically split him? Zamasu seemed immortal to penetration, ki blasts, and physical attacks, but getting sliced in half? It was never made clear if he had Boo-like regeneration for a situation like that. It could be an explanation to keep Trunks below Vegetto and keep the writing more consistent.