I had always assumed they brought cyanide pills with them just in case of disaster. I know that if I was stranded on the moon with no hope of escape, I wouldn't want to sit there for 2 or 3 days until dying of starvation. I'd rather just eat a pill and be done with it.

They probably would have died of CO2 poisoning, since the air filters were only available in limited quantity.

they could probably have just depressurized their suits and would have lost consciousness extremely quickly.

This.

Whenever asked about cyanide, the astronauts always laughed, because if they wanted to kill themselves, there were about a million ways to do so. They didn't need cyanide.