Ronan could not just see his brother being brutalized by the assassins. His emotions ran wild, sending his mind in a downward spiral.
Just like how the Phoenix clan members gained a crimson glow in their eyes when they got angry, Ronan's body also had a special mechanism that he himself did not understand at the moment, which reacted strongly to the negative and destructive emotion of anger.
'DESTROY, DESTROY, BURN THE WORLD DOWN, Little One… How dare they hurt your brother… THEY HAVE TO PAY!' A sinister voice misguided Ronan in his mind, a voice that sounded extremely convincing, whose allure Ronan could not disobey in his moment of weakness.
It was the reason behind why all those who wielded the forbidden element always became great sinners in the world, as when their emotional state regressed their primal instincts took over.