Yes, when Palps tells the tale of Plagueous and gets to the part about his apprentice killing him in his sleep, he looks forward and grins an evil, knowing sort of grin. I (and most people it seems) took this as an indication that it was Palps that killed his master after he had learned all he could.
Of course, with no indication from Lucas, it *could* only have meant that Palps just really admired Plagueous' apprentice for what he did. It is just a matter of interpretaion though.