Mana is the life energy that courses through a metaphysical circulatory system in the human body that allows it to function and perform its daily tasks to keep an individual alive. At the center is the human heart, which pumps mana through all corners of the body in the same way that it pumps blood. The amount of mana content is determined at birth and is different for every individual. Those with a high enough content can access it in a way that allows them to have external effects on the mortal world in the form of spells. This is known as witchcraft, and students of this practice spend years honing their abilities to increase their effects and knowledge to shape the mortal world. These witches range in power, with some born stronger and more talented than others. However, their are ways for individuals to get around this internal handicap to rival others. By consuming the heart of another human, an witch can absorb the mana content lying within the organ. This is done through human sacrifice, in which a witch cuts out the heart of a living victim in an ancient ritual. This adds to their mana capacity and thus their abilities, making them more powerful. Certain witches, known to laypeople as "heartbreakers" take to preying on other humans, killing them and stealing their organs to absorb their mana. Over time, a heartbreakers power accumulates due to compound interest, magnifying their abilities to put them above other rivals.
Heartbreakers are by their very nature evil and sociopathic, and see themselves as superior to their competitors. Sacrificing hundreds of people for their own gain would be standard practice for them. However, the energy they gain from regular people is miniscule, as they possess low levels of mana, forcing them to spend years at playing the long game. It stands to reason that they would hunt other witches in their craft, as their mana content is far higher than that of a normal person. Eating their hearts would provide a significant boost to their own abilities, resulting in a faster accumulation of power with a far lower body count. This accomplishes the goal of increasing their strength much sooner with the added benefit of not drawing attention to themselves by killing hundreds of people. At the very least, they would stack up on consuming regular hearts in order to take on a main rival at some point.
With all the benefits in place, what would prevent evil witches from taking this route?