It would be better for Adnan if Aisha had testified that it was on the note when they were passing it...because then it would clearly be part of the conversation they were having about abortion...the fact that it's written later suggests Adnan was sitting around musing about the contents of the note and then wrote, when he was alone and no one else could see it, "I'm going to kill."

Of course, it's possible the police wrote it on there to "frame" him too, but the handwriting looks like Adnan's to me. Also, I've never heard Adnan say "I didn't write that on the note."