An essence of a person wasn't pure because it contained faith and blessings of a higher entity.
This can be pictured by an analogy of dirty pond water.
Dirty pond water was not fit for drinking as it contained a lot of impurities and was contaminated.
The water could be thought of as essence and the impurities could be thought of as faith and blessings.
Alex's constitution would be messed up if he absorbed the faith and blessings. Not to mention, this would end up alerting the higher being and make him an enemy, since he had killed a person who was blessed by that person.
Things were far too risky, but this was the best path possible.
There were also many more ways to achieve similar results such as by making pills from different materials and then eating them.
His class was Healer and he was naturally skilled to do such a thing.
Being a Healer was a really overpowered class and Alex had slept on it for far too long.
