Cyborg Vader came up from behind Palpy and chucked him into a pit. That can HARDLY be considered "defeating" him.
As for the chosen one business; Yoda says in the novel of RoTS that the sith evolved and changed over time while the Jedi didn't, and thus the Jedi were unprepared to counter the Sith. And bringing "balance" was just misintrepreted by not meaning killing all jedi so that there were only a couple left (yoda, luke, vader, palpy). But that in itself is wrong, there were tons of Jedi that survived the great jedi purge, but I digress (little plot hole my lucas, but I can forgive it)
Dead is dead. Vader killed palpy. You can dress it anyway you want, but Vader picked the little guy up and tossed his dwarf ass and there was nothing Palps could do to stop it.
THEREFOR (again) Vader (Cyber Vader at that) was more powerful than Palps.