DM's have different roles in different places; what I want best is a clear understanding of their role for the dive we are doing today. In Little Cayman it was made clear in each breifing that we could go on our own as buddy teams or stay with the DM for a tour - and that the DM would be killing lionfish they find or are pointed out to them. .
I'm a rare poster, preferring to listen more than talk but I'm tempted to post here.
I think Qnape has hit the issue on the head - for me at least. I would propose that a possible solution could be (at least in part) about having a pre-dive brief that attempts to give each participant a clear understanding of what is going to happen INCLUDING what the expected roles of each participant will be on that particular dive. If any individual does not like what they hear, they are welcome to debate it prior to the dive and / or make a decision whether they still feel comfortable diving giving that knowledge. (of course, there may be language barriers which is a whole new kettle of fish...) As with what happened with Qnape, this could be when a DM mentions that he is an avowed Lionfish Slayer and will be ridding the world of this scourge where ever possible on that particular dive. Ultimately, the DM needs to be showing the guests the experience they want (at least what is within his power to control) so if they aren't keen for a killing, he'll just have to put away his spear for another day.
Though I don't get the sense that the OP ever suggested otherwise, I am a firm believer that each buddy pair is ultimately responsible for themselves and that a DM's role is to guide his guests safely through a dive site given his knowledge about the site and his understanding of the capabilities of the divers he is guiding weighed against the existing conditions at the site. This does not mean DMs are there to babysit people and replace common sense and proper training. (I won't get into the times I've seen dive operators allow people to dive a site or dive in conditions that were clearly beyond the capability and training of a guest as this is a separate issue - let's assume for this argument that the DM / dive operator has appropriate processes in place to ensure that incapable / undertrained guests are not diving.)
We all know that there exists a belief out there for a number of people that DMs are underwater lifeguards. I don't know where the DM reliance comes from (maybe from the numerous lawsuits out there and from the same place where someone thought that they could sue McDonalds for handing them a hot cup of coffee). I think this view slowly goes away the more experience a person has - it certainly did for me, especially when I realised that DMs are not perfect and sometimes make stupid decisions, like the rest of us mortals.
The same discussion which ensures a DM has the full support of his dive group for his Lionfish Slaying could go a long way in addressing the DM as a Lifeguard misconception. The message needs to be said in plain speech and repeated over and over to get through the thick skulls out there. While it may be frustrating to many DMs (and many non-DMs for that matter) to have to state something that seems like motherhood and apple pie to them, it might eliminate a lot of pain if it was brought up prior to each dive. In effect, put up a "NO LIFEGUARD ON DUTY" sign - if it works on the beach / pool, it might work in the water.
I think clear discussions / disclosure like this should occur in any situation where there is a well-known existence of differing opinion (eg what is the role of the DM) or where what the DM is doing digresses notably from the common definition of a dive master / guide (or where there is no common understanding of a situation / plan)...
I am not a DM, I am not an expert, just a hobby diver, so I'm sure my opinion is based on incomplete information and fraught with reasons why it can't be implemented as simply as I state it, but maybe it at least provides a bit of food for thought....
In the meantime, here's a little video of mine about my own surprising encounter with a very friendly moray eel for your enjoyment.
YouTube - Close Encounter with a Moray Eel