Ruin was influenced in his youth by a very traumatic event: The execution of his family, while he himself was present. This gave him one driving motive throughout his life: Revenge. This impetus was strengthened by the mutations caused by his Vamprirism, and it gave him the devotion to create and launch an armed crusade against his family's murderer. Most people would not have taken their emotions to such an extreme, but Ruin was a special case. His entire life became revolved around this one point, he even changed his name to reflect how he felt: Ruined, by his parent's death; Ruined, by his affliction of Vampirism; Ruined, by his obsession with revenge. Ruin was broken, however his behaviour had become easily predictable, assuming one knew his motive: The Death of Tergen.
Ruin was born to a family who lived in a village of Gem Cutters. His childhood was plagued by the threat of Vampires, a threat which eventually lead to the village's massacre at the hands of Tergen and his army. This massacre provided the death of his family, and his escape from everything he had ever known. At some point he was infected with vampirism.
Ruin went on to change his name and raise an army of vampires. His name became feared throughout the north, where he eventually attacked Tergen's town. His army was destroyed, but Ruin managed to confront Tergen while he was sleeping. The two fought, but Tergen was victorious and the vampire chief finally died; However, this was not before he infected Tergen with the disease of vampirism. Perhaps this was always his motive: He felt that the loss of his family had infected him with vampirism, and since Tergen was certainly to blame for his family's massacre, he attached Tergen to his other major qualm. Therefore, giving Tergen the curse of the vampire was simply him returning a "favour".