Under the empowerment of a vengeful heart and adrenaline, human brains at that time would only think about how to kill you, not considering whether they themselves would get hurt or not. They couldn't even feel pain; before killing the opponent, they would only have a trembling sense of excitement.
At that moment, a human is no longer just a human, but rather a true terror— the Upright Ape.
They would remind all those beasts that thought they could intimidate humans how in ancient times, they were dominated and hunted by the terrifying Upright Apes.
Now, An Dan is clearly akin to an injured Upright Ape determined to take ferocious revenge. Once his eyes harbor only resolute decisions of life or death, questions like "is it necessary," "is it worth it," "what's the cost" are no longer in his considerations.
In short, as long as he could take a chunk out of Emperor Yin, he'd do whatever it takes.
