Space Viking by H. Beam Piper [Federation Series #4]

SPACE VIKING is set in the distant future, when civilization has collapsed into feudalism and anarchy. The murder of his wife-to-be on their marriage day sets Lucas Trask on a quest for revenge. With the purchase of a space ship, Trask enters the world of Space Vikings, and begins raiding worlds while simultaneously searching for his beloved's killer. Realizing that raiding is a waste of resources, Trask begins to trade, begins to build a galactic empire, and begins to face the despotic power of his wife's killer.