.... but the dive in question was at a resort where "resort certifications" and new PADI certifications abound. Part of the sales pitch is safe diving under the watchful eye of experienced DM's....
If there was a resort course diver present, if that "DM" was assigned to be his supervisor and buddy, then yes- he was overtasked and not paying close enough attention per any standard.
If this DM was the dive boat crewman who was in the water with your group, he has only the responsibility as directed by the resort and promised to you, a certified diver.
Do not read more into the DM title than you should. DM's in paradise may or may not hold certifications of any kind, but even if they are illiterate, they are likely better in control of their herd of cats than anyone working as a DM back home with 15 patches and c-cards.
If you ask if they are certified as a DM, Oxygen use, or Rescue Diver, you'll get an answer. The real question is... what is their assigned responsibility?
You bring up a new phenomenon in recreational warm water diving- divers are becoming more and more focused on Lionfish hunting. There was a time when noob divers wanted to bring their u/w cameras on their open water portion of their cert dives. Many new divers take on too much to handle, and very early on.
DMs hunting not only might hamper their observational skills in regards to their cat herding duties, but they diminish the enjoyment of the guest divers because they are not showing the paying divers all of the interesting cool stuff.
Contrary to many posters here who eschew the assistance of a DM in spotting critters, I absolutely follow them like a dog. Sometimes, I find as much or more than they do, but 2 more trained eyes are always better than just me alone.
As for divers carrying a pointed stick in my vicinity? A good DM, if he must, I wouldn't be afraid. A guest diver with a three-pronged skewer? I'll be waiting... way over there.