An athame is a magical item that is created to reflect and direct psychic energy. To create this tool, one cannot simply enchant ordinary weapons to give them magical properties. Rather, these items must be built from the ground up, through special materials, ingredients, and rituals. The process can take months or even years to complete, but result in an item reflects the emotions of the user. Athame can take various forms, such as knives and swords and other weapons, or mundane items like a chalice or a book. As the athame absorbs the pshyic energies of its wielder through constant use, it grows more powerful. Through this symbiotic relationship, a great and noble hero can produce a powerful weapon imbued with his positive energies, such as Excalibur. Likewise, a notoriously evil warmonger can create a negatively charged item with evil properties that is geared toward causing pain and suffering.
Even though athame reflects the psychic energies of its user, an item wielded by an evil character always maintains its stain of corruption. The negative energy that the item absorbs cannot be changed to a positive state. It retains its evil properties even when being wielded by a noble character. In fact, the item can begin to corrupt that character's nobility, turning him darker until he begins to reflect the negative energy the item imbues. This attribute doesn't work in reverse. An evil character who uses a weapon previously owned by a hero will not take on heroic qualities and reflect the positive energy of that weapon. Instead, that weapon becomes corrupted, and its declining state into a darker version of itself cannot be reversed.
It would be easy to say that negative energies are stronger than positive ones, but this is stupid. Powerful emotions exist on both sides of the spectrum, and can lead to characters who feel them strongly. There is no difference in commitment between a Hitler or a Mother Theresa. Why would this transfer of energy only occur in one direction?