I find most junglers have a more defined role of what they should be doing during team fights, with Nocturne being an execption. You can dive the carry with your ultimate, but you leave your team with out their bruiser and deep in the enemy line. On the other hand if you sit back and protect your carries you are subject to range damage and can't contribute that much damage of your own. So my question is what should Nocturne's focus be during team fights?
Nocturne to me is an "anti-carry". Make sure to play him in a team that can handle you not being around them in a teamfight. After you dispatch their squishy carry, fear the most disruptive member of their team to minimize their impact and then focus on the next highest priority target.
If you build him full out dps then yes you should most likely zerg their carries depending a bit on the situation of course. If you go a bit more off-tankish build then go for support in the beginning and ulti their carry when they run away with low hp.
Nocturne's main job is to kill the carries. What you should do is jump on their carry if you can. If they are grouped up, wait a little for ultimate's and summoner's to be used and then clean up the carries afterwards.
It all depends on how you choose to build him, you can go a sort of offtank initiater or pure dps gankie build.
With him what you do is ult and focus the AD or AP carry so you can quickly shut down most of their damage output so that u can secure an easy team fight for your team...
Nocture is used to initiate on the carry and take punishment.
Nocturne is used to jump on their most dangerous person, someone like their ad carry. If they are careless then you can just annihilate their ranged ad, and their highest damage dealer gets eliminated, making it a 5v4
Nocturne is an assassin type jungler. He jumps on a squishy with his ult, keeps himself safe with his shield kill them and try flashing away. In the mid to late game (after laning phase), I see good nocturnes take the role of split pushing a little bit as his ultimate allows him to have better map presence than most champs, tf/pantheon/shen not withstanding, so he can force an enemy to stop his push and initiate a teamfight 4v5.