How could this man possibly be "not guilty?"?

A friend of mine’s father was murdered last week by a teenage psychopath who was looking for someone to kill in the park. Police found a note in his wallet (see attachment at the bottom) that perfectly described his very detailed plans of finding “prey” in the park and to “enjoy the killing”. It perfectly matched what happened to the man he murdered. There were also several eyewitnesses who saw him in the park and could identify him.

Yet he’s pleading not guilty. How is this possible? How likely is it that he will be found not guilty?