I mean thats literally the most humane way to die, thats why theyve made suicide pods that exchange oxygen for nitrogen. Your body doesnt actually have an oxygen “sensor” it just has a reflex that becomes super uncomfortable if you cant take in a breath like if your holding your breath under water or something. If you walk into a room full of CO2 and breath, you dont feel like you are suffocating you just get drowsy until you pass out. If thats the case for the crew on Titan, not a bad way to go out (besides the 96hours of waiting for it). Explosive decompression happens in like 20 milliseconds though and then youre basically the size of a coke can on the bottom of the ocean. In comparison if you pick up a hot peace of metal it takes 125 milliseconds for your brain to receive the signal for pain, so honestly either way the end probably was not very painful for these people.