This is a tough issue. Depending on why you think they are dangerous and how you share your thoughts, you may become the diver everyone avoids. You should discuss the issue with other divers. At some point, discuss your issues with the "dangerous diver". Maybe this is an misunderstanding. Maybe the "dangerous diver" will change. Either way, I feel everyone should be aware. If you lack confidence with any diver, you should not dive with them.
I had an issue with an AI. I was diving on a boat that was chartered by my LDS. So we had a group of 20 divers who knew each other on the boat. The AI leading my group of four divers was on nitrox 28 and dove a profile for nitrox. The rest of his group was on air. Two of the group, (his son and his son's friend) were just OW certified using gauges. I used an air computer so I was aware we were into deco. The dive was on the U352, depth around 90-115. Since the entire boat started their dives around the same time, you would think we would have come up with the entire group. We started our ascent roughly five minutes after the rest of the boat. We were on the wreck, everyone else was on the hang line. That should have indicated we were longer than everyone else. My computer was 4 minutes over the NDL when I got the AI's attention to tell him I was ascending without him. He brought the rest of the group up, passing me on the anchor line. My computer wanted 30 ft/min ascent. They were faster than 60 ft/min.
Luckily, no one was hurt.
When we got on the boat, I mentioned I was 4 minutes over and I was ten feet shallower, he indicated he had 6 minutes more bottom time. Another instructor reminded him he was on nitrox, extending his bottom time but not his sons, how was on air. He seemed to be not concerned. To me, this is a diver to avoid. Even scarier, he is an AI. Following this and other incidents, I suspect the shop would not support him becoming an instructor. I have not asked but I wonder if I ended his career. This diver has asthma, dives with an inhaler incase he gets an attack. The prior day, he did not fully open his tank, at 20 feet it was hard to breath. He surfaced, pulled the inhaler out of his BC, took a hit and started to descend. The boat captain had a tough time getting him back in the boat before the captain advised him he could not dive. The same diver has lied about his asthma condition to get a doctor to clear him for diving.
I ask you, should anyone dive with this guy? Only if you are experienced and setup to dive solo.
Sorry for the long post. You are not the first to be concerned by a diver, you will not be the last. I would talk it out. Don't hide your concerns.
Tom