He isn't mentally retarted, just partially paralyzed from an eastern medicne treatment gone bad.  He tried to kill himself manytimes.  Feeling suicidal, he thought it would be unfair to die alone, so he decided to take some with him.  125 died, but he didn't.

While improvements are being made, most Koreans are not kind toward handicaps and looks down at them as if they are sub-human.  I have seen people yelling and cursing at handicaps because they were too slow to get off the bus.  While I don't condone what he did, things must have been unbearable for him.  He probably would have preferred being fully paralyzed since he wouldn't have been humiliated everyday as he tried to go about his daily business as best as he could with his half-paralyzed body.