That's what I had thought too, but thinking on it a bit more, there's more meaning to it. It's the same way his mother had tried killing him when he was a child. I kind of think that when he was suffocated, it may have brought him back to that time, which would cause a large amount of stress, helplessness, and fear. That's exactly what his mom wanted. Plus, if he did die in the fire, we wouldn't have it as to were he was calling for Bum in his hospital bed every night. It showed that he did die the most painful death: alone, scared, and helpless. His mother got what she wanted.
Ah really? The burns when he was pulled out seemed pretty severe since he was in there longer than the officer. Plus his ashes and the old lady admitting to killing him. What do you think happened to Sangwoo?
What if it's all a conspiracy and the police made it seem like Sangwoo was dead to stop the protests. XDD what a twist
If Sangwoo had died by fire explosion, it was more acceptable than dying by a lame pillow, helplessly.