He had hardly been a member for more than a couple of years.
Entryism. Moribund local party, run by a long-serving, loyal but exhausted old salt who is simultaneously chair, membership secretary, treasurer and campaigns officer, with at best nominal help from a revolving cast of students and transient hipsters.
Suddenly, over the horizon comes an organiser. Someone with experience in other groups - scouts, say - bringing also a wife and children who are also keen and motivated. Suddenly, there's a meeting which is quorate! They bring some of their friends, too, so there is stuff happening!
The Labour Party learnt the hard way that those bright, keen new arrivals are usually Trotskyites, and their objective is to use the respectability of a moribund local party to further their political agenda. Which is bad, probably, but amongst the groupuscles of the hard left it sort-of doesn't matter.
David Challenor didn't have different views on the role of Leninist Vanguard Parties in the early stages of a revolution. He was a child abuser, who wanted to get a cover for abusing children. The respectable family man, who's a scout leader and local politician? What could be better?