They don't blow up, but they will create a bigass torch that will spin them around some. You're not getting blown into pieces, but you're going to be going into the er with some bbq'd body parts.

Yeah nah, that's not the way. Sure people can get away with it, but it's really not eliminating the hazards.

• https://www.fnqsafetygroup.org.au/wp-content/uploads/2015/12/cutting-empty-drums.pdf
• https://www.worksafe.vic.gov.au/safety-alerts/explosions-when-cutting-used-fuel-tanks-and-metal-drums

There's a bunch of non-obvious hazards which exist around cutting into containers which have held fuels, including but not limited to:

• An empty propane container could have an gas+air mixture inside it in proportions which could explode.
• Any liquid residue in an empty fuel container can be vapourised by the heat of the cutting, creating a fuel+air mix which can spontaneously explode from the pressure and heat, even without sparks (think: diesel engine principle but instead of a 0.25L cylinder it's a 200L cylinder).
• An empty gas canister can be contaminated with heavier oils/waxes that could build up over the years of refilling cycles, creating the same hazard as noted for empty liquid fuel containers.

Methods to reduce the hazard include:

• Thoroughly washing the inside of the container before cutting (access can be difficult though).
• Filling with water and cutting while full of water. (Container musn't be sealed, and the cutting method must be safe to use in a wet environment, and further hazards such as introducing oxygen into the container need to be considered).

Because the tank was almost empty. Who knows what happens if it's full, or overpressured, or corroded or something. Hiding behind a solid object would be my move as well.

BLEVE

Pressurised gas is a different story, and liquid pressurised gas is even worse still.

Basically a bomb if they are in a fire.