This is very sad and very avoidable. It sounds like the sub was filled with extra personnel-- shipyard workers or staff riders-- and many were injured by Freon inhalation?
Twenty die in Russian nuclear sub accident | International | Reuters
I didn't think any navy used those systems anymore. I'm out of date, but I think even the U.S. Navy's surface fleet has disabled or removed their space fire-suppression systems for just this reason. I don't know what's in a Russian fire-suppression system, but if it's R12 or similar chlorofluorocarbons then open flames can convert the Freon to phosgene. So not only is there no oxygen to breathe but the remaining atmosphere might be highly poisonous.
Even for the U.S. Navy, let alone the Russians, sea trials are usually an uninterrupted series of minor crises. Everyone's exhausted from the last month of trying to get ready, many have been up for 24-36 hours by the time sea trials starts, the crew has lost their proficiency at just about everything, every space and every major system has been torn apart for months or even years so hardly anyone can remember where anything is or how it's supposed to work, and a whole bunch of tests & evolutions have to be accomplished on a rigorous schedule that doesn't survive the first day of "operations". At least a third of the crew is brand-new, most of them have never taken this particular boat to sea before, some of them have never been to sea, and the rest of them haven't been to sea in months. The boat is usually filthy from all the industrial work and even several days of dedicated cleaning would just begin to remove the dust & debris that's in every crevice. Flammable trash is left onboard by accident (tucked away where no one noticed it) and things have been put back together wrong. There's a bunch of new gear that no one's ever really worked with before, and everyone's mostly forgotten what the old gear is supposed to sound/smell like when it's running. There are so many extra people on board (120 crew plus 20-30 shipyard workers & staff riders) that any testing will jam the area with bodies who usually don't belong there. Even if the crew is capable and safe, there's still relentless pressure to cut corners and catch up to the schedule.
U.S. submarine crews can train in a shore fire/flooding simulator but everyone is really busy doing "more important" things and may not get anyone through the training, let alone proficient. They're also required to conduct at least a 24-hour "fast cruise" (moored fast to the pier) with the hatches shut and everything running to simulate underway conditions. Ideally most of the problems will make themselves known on fast cruise before the boat submerges. The crew also runs a lot of drills to make sure that everyone can find the damage-control equipment and that it actually still works. Heck, people would be so rusty that sometimes it took a half-hour of practice to get the whole crew to don their emergency air-breathing masks (EABs) within the required minute.
The article says that most of those killed were shipyard workers. U.S. shipyard workers don't get any damage-control experience. They all get a brief on where to find the emergency gear and how to use it, but they have only a fraction of the proficiency of the already inexperienced active-duty crew.
The Russians don't have the money or the culture for extensive training. I've been told that a significant number of the crew (perhaps from other republics) can barely speak Russian, let alone read it. Others are conscripts with very limited knowledge beyond the trained-monkey stage. The Russian Navy lacks a cadre of non-commissioned officers who are skilled at maintenance, training, initiative, & leadership. Having dealt with the U.S. system, I can only imagine what a disorganized pigsty the AKULA must have been.
So in this environment, the testing starts. There might not have even been a fire. Someone might have accidentally triggered the fire-suppression system, or something failed and it triggered itself. Or everyone's clustered around a piece of gear that starts to smolder, or some unattended system has a problem and starts smoking. Everyone's slow or uncoordinated or in the way, everyone's shouting and coughing and blinded, no one can get to the emergency gear, and some people either freeze or panic. Then everyone is trying to find an emergency air-breathing mask, let alone an extinguisher, and a lot of people have never had to find either one before. If indeed the masks were where they were supposed to be, and if the emergency air-breathing system is working correctly, and if they know how to use it. Some conscript knows only that "if a fire breaks out, flip this switch" and the fire-suppression system is activated. Or a supervisor triggers one of the compartment systems because he thinks the situation is out of control.
I'd like to think that a U.S. submariner would perform better in this type of casualty but experience suggests otherwise. Testing on instrumented submarine hulls has shown that a few cups of oil catching fire in a bilge can smoke out the engineroom and raise temperatures above 150 degrees within five minutes. The U.S. Navy tries to avoid these situations in the first place and it's why sea trials are frequently delayed amid prolonged cleaning, training, & requalification. Prevention is far more effective than damage control-- and automated systems don't always find the problem, let alone stop it.
U.S. submariners are trained that when the emergency is discovered (or when it finds you), then you're supposed to sound the alarm and make a report before you're incapacitated. Even if it's a fire you don't evacuate right away. You're expected to don an EAB, grab an extinguisher or a fire hose, and buy your shipmates the time they need to get their damage-control party properly equipped and on the scene. If you can't get the situation under control within a minute then a bunch of other people will have to risk their lives trying to save the crew.
Years ago I heard that U.S. shipyard employees and some contractors got paid triple wages, 24 hours per day, for going on submarine sea trials. The younger guys would volunteer for sea trials expecting to get rich. The older guys (who perhaps had volunteered before) would stand back and let the younger ones learn why it paid so well.