Like most every evil king in every story, in the end Pelias gets what's coming to him. We guess it's payback for the way he stole power from his half-brother, Aeson, and then went back on his word to Jason by refusing to give up the throne. It's interesting the way that the myth of Jason veers from the formula a little bit.
When Medea tricks Pelias's own daughters into killing him, without even asking Jason first it definitely puts the story into less charted waters. Usually, it's the male hero who slays the bad guy. In Jason's case however, his wife convinces a gaggle of young girls to do it. Of course, Pelias's murder ends up being a curse on Jason's life. Because of it, the hero never gains his throne and never lives up to everybody's great expectations. So, in the end, does Pelias actually win?