Although reiving was specifically a form of theft, its practitioners were also frequently at feud with each other. Reiving is the simplest form of lethal intergroup violence. Hate was not the motivator. Reiving, like all raiding, requires emotional distance. You don't rip off your own people.
Feud is a different story.
Here we encounter fighting for its own sake. Your reputation would help protect you. You strutted a lot, boasted a lot, and, in a land where oath-breaking and betrayal were frequent, you reacted violently to slights and insults. If folks locked up, killed, or interfered with your kinsmen, you, as head of a grayne, took immediate retaliation in kind. A member of one family might, defending his property, kill a member of another family.