Final answer:
Adults think Dennis Brewer is strange because of his peculiar behavior and seemingly fragmented way of thinking, as suggested by the various odd elements and historic references within the narrative.
Step-by-step explanation:
The narrative in the story 'Murder' seems to illustrate that adults find Dennis Brewer strange due to a combination of his actions and possible mental state.
The collage of peculiar elements such as the father's 'hairy face,' Betty Byrne's 'lemon platt,' and the distorted snatches of song contribute to the portrayal of Dennis as an eccentric individual.
Further, the accounts of historic crimes, like the case of Jean Gianini, where a seemingly straightforward act of murder is complicated by questions of the murderer's sanity and intent, add to the thematic complexity around perceptions of strangeness and criminality.