In case you read Countdown, Nero makes a 180° turn from being Spock's friend to a guy who generally distrusts Vulcans and suddenly threatens Spock with what he will do to him if he should fail. It made absolutely no sense, so let's call it the 'Shinzon moment': why not go after the folks who really messed it up, which is in both cases the Romulan government.
That's why your idea about the planetkiller is better. If the movie is too crammed for a well-developed humanoid villain, choose a good ol' non-humanoid sentient being which needs less screen-time (at least I got this impression from previous Trek incarnations and beings like V'Ger or the Horta or the Excalbians) to work fine.
Last edited by horatio : 06-03-2009 at 12:49 PM.