The Sucide Note is the when we see Kyle Brofloski's suicide note. The Note says:
Dear, the Good People of South Park,
I went into Wendy's room. She wasn't home. I looked under the bed and sure enough, like the rumors said, a case holding a gun was there. I'll make Kenny and Stan and everyone else pay. I knew exactly what I would do. I knew Kenny gave up immortality. I knew Stan wasn't immortal either. I was ready, a gun, a plan and an insane mind, torn by all the ridicule, hate and relationship issues I've had to endure. Kenny made a fatal mistake rejecting me and making that damn three way. And Stan should die for hurting me 3 years ago. Now that I think about it, Bebe and Tammy should die too for their part in making me do this.
God, South Park will pay. It will pay for what it made me endure. My death list is on the other side of this note. Look at it and check off who all I killed. If they are all dead, then thank the Lord. I've saved kids from humiliation and torment. Look at the list now for those who are ready.
The chapter also shows the first two lines of The Shooting which are:
The shooter looked at the school. He pulled the gun out of his bag and put it in his pocket; he rolled his shirt over it so no one could see it.
He walked into the school...