He wanted to keep his awareness and senses, so didn't want to activate the Blood Berserker extension of his Bloodlust Characteristic, which left him with his other Bloodlust Characteristic extension.
The Blood Burn extension, which would burn his blood, vitality, life force and energy in order to increase his strength, was the only thing he could resort to in his current situation, unless he gave up and restarted Stage 3 from the beginning.
If he used it in reality, he could lose years of life or could even push himself to death, all while damaging his body and cultivation to boost his strength.
But in the dungeon, he didn't have to worry about the physical damage, and he felt that his mental fortitude had increased, so he would be able to handle the mental and spiritual backlash that he would experience.