I work with cognitively disabled people, including those with Down's, and the behavior of the young man is typical when one understands how someone with his affliction "views things." There are different levels of Down's, from moderate to severe, and some with the condition can live very productive and independent lives (recall the young actor who appeared in that one TV show called, I think, "Life Goes On"). Because of the brain's inability to process things, they often become highly focused on something and anything out of the "norm" of it (as they see it) brings confusion and trepidation. For example, with several of the Down's people I work with, they have very set patterns, even in the way they eat, and if this is changed it disrupts their "comfort zone" and they can become very aggitated. It's like having a security rug pulled out from under you. In the case of the young man at the festival, and I'm hypothesizing here, to him KLS was Maggie Evans, and the show was probably "reality" to him. When she, as Maggie, left the show, it disrupted life for him, and thus her "betrayal." Of course, it might have been some other "issue," but that's how he saw things; he can't help it.
But I do agree that his aide should have been with him at all times. He was in a strange place with nothing familiar to him, and a greater risk was not what happened on stage, but if he should have become lost and totally disorientated, causing panic. Imagine him, in confusion, walking out the door of the hotel into the streets of whatever city this was in (places like New York and Los Angeles can be overwhelming for "normal" folk, let alone those with a cognitive disability). Turn your back for a second and it can happen. We never, ever, leave those in our charge out of eyeshot for any reason.
Gerard