From UOGuide:
Until the Stygian Abyss expansion, all Slayers did double damage to applicable monsters. However, following that expansion, Lesser Slayers did triple damage, and Super Slayers did double damage. The manufacturers stated explicitly that this was intended to increase the value of the Lesser Slayers, which were increasingly looked down upon by players (with a few notable exceptions, such as Scorpion Slayer, Spider Slayer, and Dragon Slayer, all of which had particular uses).