Lore
Specters usually arise spontaneously from those who die a violent death. A specter is always created from someone who is jealous of the living for continuing on while they have died. By becoming a specter, they let go of their empathy for living things and decide to destroy all life they come across. As a result, most specters come from people who died while consumed by great anger. Very few people would become a specter if they were thinking clearly. Sometimes, a necromancer guides a soul to become a specter, but the soul must always have some envy or anger towards the living.
Sometimes specters work with living monsters or necromancers on the condition that they destroy more life than they save. Sometimes they also have a preference for destroying a certain group or type of creature.
Most often, specters haunt battlefields, dark temples, or places where people died in an unfair and painful way. Hospices must be careful when particularly bitter people die of old age, and usually keep a cleric or paladin around just in case.