Morale was crushed
There are many limits that people indirectly impose on themselves; after all, morality exists for that. But for Dante, what he saw wasn't just a "breaking of his moral limits"; it was like taking his entire existence and crushing it.
That scene... it made him have flashes of things he never wanted to witness in his life. Certain images were so brutal that he couldn't even speak them aloud.
Axilen, Ethan's fiancée, was in a near-death state as her body continued to be hurt while he dragged her by her hair.
When Dante saw that... a switch flipped.
A big red button in his mind was starting to be pressed.
He positioned himself as the third person in the situation, he looked at that deplorable scene and simply couldn't see himself there anymore.
Dante, someone who fights tirelessly to protect those he loves, but is weak at times.