During his hallucination in the grove, Simon speaks to the beast and it tells him that, in the end, the beast is inside of all the boys. Their fear and their immaturity and their distance from what they considered civilization, etc., have all conspired to make this beast stronger and stronger. Simon begins to understand how this is happening and rushes out from the grove in order to tell the boys.
Once he comes out, he runs right into the middle of one of the ritual play hunts and is quickly killed by the already excited boys, thinking that he is the beast itself. You might classify this as a somewhat poor reaction!