Bxx1, "freon" (a generic term that covers a lot of different manufacturers and gases, like you might use the term "coke" or "kleenex" for instance) displaces oxygen in your lungs. Once you breath it in it's really really hard to get it out, so you suffocate, or die in your sleep as the case may be... The air mattress leaked freon overnight, displacing the oxygen in the room and the guys lungs. I guess similiar results could be had by filling your air mattress from a hose hooked to the exhaust pipe of your car? Yo, Darwin rules!
Rumor has it, I've never personally confirmed it, that freon when exposed to a flame, creates very deadly phosgene gas. So, when that VW was on fire on the side of the road and I didn't have an extinguisher handy, I used a 30 lb cylinder of freon R-12 I had with me. Turned the bottle upside down, shot the fire with liquid freon, snuffed it out in a heart beat (no oxygen you see) and then ran like hell from the monster "phosgene gas" cloud I'm afraid I might have created...
Same principle with Halon Extinguishers, the gas will STILL suffocate you (and put out the fire) but apparently doesn't react badly to a fire to create a deadly gas.