Did Worf murder Mendon? The latter repeatedly annoyed the Klingon in "A Matter of Honor." After the episode, Mendon is never heard from again, though he had an important role in saving the ship from destruction. Did he meet a fate similar to Duras? (Perhaps the Enterprise covered it up.)
All signs point to "no".
Mendon was part of the 'Officer Exchange Programme". After the episode ended, he's not heard from again (at least in the main canon of TV shows and films) but it's far more likely that he simply returned to his own ship rather than being killed by Worf and quietly disposed of.
You may wish to note that Riker was also off of his own ship for a similar amount of time.
In the EU canon, Mendon (newly promoted) makes a reapparance in TNG: Rogue Saucer. Suffice to say he's hale and hearty and thoroughly un-murdered.
A few seconds later, Picard found himself looking at a young Benzite lieutenant with blue skin and a breathing apparatus attached to his chest. With a soft hiss, the apparatus released gases into his nostrils, and his fishlike face beamed with delight. “Hello, Captain Picard, do you remember me? Perhaps you will mistake me for someone else of my geostructure.”
The captain wagged his finger thoughtfully like an old schoolteacher. “Mendon, isn’t it? Or should I say Lieutenant Mendon. Life in Starfleet must be agreeing with you.”