Kain meets the barkeep of Ziegsturhl during the very first scene of Blood Omen, inside the tavern, while Kain is still alive. The barkeep rudely sends Kain outside saying that the tavern is closing, and despite being promised gold for a chance to stay, claims that "things that come in the night" are too dangerous to take the risk.
It's unknown whether the barkeep was in league with Kain's killers or not, but it was his inhospitality that forced Kain into their hands. Later, as a vampire, Kain can visit Ziegsturhl again and find the barkeep behind the counter. Though it's optional, one may choose to take revenge on the barkeep.
He can be killed by any ranged spell or artifact (Flay is the earliest available option). Using Wolf Form allows jumping over the counter and mauling him to death. If, however, one believes that a quick death is too good for the barkeep, there is another option.
Behind the barkeep, past the counter, is a door with a portal: a secret backdoor to a Spirit Forge. Using Control Mind / Spirit Wrack on the barkeep allows sending him through the portal and sacrificing him to the Forge. Not only will the barkeep suffer a gruesome death, but Kain will also be awarded 99 Flay devices.