I believe these two groups are in it together. The unicorns lure the target, and the hitmen take them out. If the hitman fails, the poison of the unicorn's horns could finish the job. It's an awful use for these poor unicorns - many targets follow the unicorns because they want the unicorn's horn. And sometimes they will achieve their goal, and the unicorn will be basically rendered powerless. So, basically, it's just another case of hitmen screwing over the hapless unicorns. Although there are sure to be times when an especially evil unicorn will screw over the hitman.