Sentence:  What was starting to unsettle him, to frighten him, was the idea that Merry was less horrified now than curious, and soon he himself became <WSD>obsessed</WSD>, though not, like her, by the self-immolators in Vietnam but by the change of demeanor of his eleven-year-old. 
Meaning of the word marked with <WSD> tags:  Intensely preoccupied (with) or (by) a given topic or emotion; driven by a specified obsession.