I’m assuming he plans to kill Rumple by stabbing him with the dagger. I speculated recently that Hook would in fact stab him, but it wouldn’t kill him, and Belle’s love would somehow save him, and then the Dark One curse will cease to exist because it can’t go back into Rumple, and if Rumple’s not dead, then it can’t move on to someone new either. If something like that happened, Hook would inadvertantly save Rumple’s soul whilst attempting to kill him, which would be sort of poetic…and would piss Hook right off…the moment he’s been plotting for almost 300 years, and it all goes completely wrong…talk about unsatisfying revenge.
Or, it could teach Hook a good lesson about how good always defeats evil, and which side of the fence he’d be better served to be standing on.