Space Viking by H. Beam Piper

A galactic war has left the Terran Federation in ruins. Formerly civilized planets have decivilized into barbarism. Space Vikings roam the wreckage, plundering and killing for gain. Lord Lucas Trask of Traskon was no admirer of the Space Vikings, but when murder takes his wife on his wedding day, Trask trades everything he has for his own Space Viking ship and sets out on a galaxy-wide quest for revenge.