Eric needed to consume the beast mana in order to gain the points for a tier upgrade, which in turn gave a massive boost to his stats. Only through the consumption could he also awaken by filling the 100 mana consumption point for the Awakening bar.
But this was beast mana, not a human one, and he knew full well the consequences of consuming beast mana: it would always leave a toll. He'd get a sudden boost on all his stats should he consume the beast's mana, and show inhuman strength and speed that cannot be contained in his current Willpower stat.
He was not willing to make the others see or feel an inhuman strength from him, one greater than all of theirs combined. A sudden, inexplicable surge in his stats after a beast's death would be a question he couldn't answer. They would all notice.
Also, beast mana uses up a little of the human mana he's stored for hunger, and he planned to conserve that mana for as long as possible even though it was on the verge of being used up.
