Any answer besides "Ukrainian missile" is going to be even more embarrassing for Russia. At least a missile strike is enemy action. Any alternative explanation for the sinking of your flagship is just incompetence.
Fires on ships are very harsh and dangerous environments. The US recently lost an entire carrier while in drydock with the maximum resources available, not even out at sea. Once it gets going, it's extraordinarily difficult to get under control.
That fire was set by sabotage and the saboteur disabled many of the fire control systems before setting the fire[0]. Embarrassing for sure, but not quite maximum resources.
Damage control was turned off because of maintenance.[0] A single 19-year-old junior sailor being able to single-handedly cause billions of dollars of damage in normal conditions would have been a very bad design flaw itself, anyway, but from what I've read, that wasn't the case here.
Anyway, until the trial occurs, I wouldn't be so hasty to judge whether the sailor was actually responsible for deliberately setting the fire, or just the scapegoat for a multifaceted failure.
To be fair, the ship was considered to be not worth repairing, but did not sink. It could have been repaired. But your point is accurate that it in drydock and it is embarrassing that the fire couldn't be put out sooner.
Any insight as to why fires on ships are hard to control? I hear it and believe. On the other hand I've been inside of military ships before and they are partitioned, with lots of metal doors to section off and contain damage, so I don't understand the difficulty.
Flammable materials like paint, lubricants, furniture, sailors are quite common inside those metal rooms. I have read of cases where the fires were so hot as to cause combustion through the bulkheads by radiant heat, regardless of the compartments being shut to section off the fires. The weight of water from firefighting efforts can cause stability issues, especially if it starts to slosh and compound any damage-related listing.
On top of these reasons, the segmented and compartmented nature of ships tends to intensify and concentrate heat. The segments that can burn, will burn hotter, much hotter, especially if they can establish a direction of airflow.
Flashover is always a major concern, everything emits flammable gasses nowadays when heated to sufficient temperature, and ship fires absolutely reach sufficient temperature readily.
I highly encourage everyone to watch this video, this is not just important on this but it's really important to understand the rapidity of fire progression in general. It's worth a watch from the start but I'm going to link to the point where it gets colorful. Imagine this is all taking place in a steel blast furnace with locked bulkheads - it's literally a matter of 30 seconds between "my trash can fire is out of control" and "everything in the room is literally on fire".
