Based on Sam's explanation, the reason he had a lot of ability instead of just one or two was that he absorbed a lot of people, including the power of the previous king before Stefan. The only reason that the late king, Sam's father, survived was that he was too powerful that Sam couldn't absorb him completely.
Still, because of this, the late king slowly withered until he passed away. It was safe to say Sam killed his father slowly and painfully.
That was the reason Sam didn't even consider absorbing the core inside me. It was risky, and he didn't want to risk that. He was just left with no choice and put his heart into it. Luckily, he was successful.
But now, he had those voices inside his head. Although he said they were great company, as if having a built-in political advisor, it was still... shocking.
