The problem was that Jim’s post contained no actual humor. Saying essentially “hey maybe your seven year old wanted some old creep to try to fuck her, ever think of that? :haw:” isn’t injecting humor into a tense situation, it’s injecting grossness into an already gross situation and making light of the most serious, unfunny element of the whole story.
For an example of how to deftly use humor in a non-inappropriate way, see kayaker’s post in the same thread:
Both touch on the same uncomfortable topic, but the difference in the two posts is night and day. What’s being made light of here is the neighbor’s grossness, in an obviously absurd way. While the OP may not be in a place to appreciate any humor at all, it’s clear the intent was to put down the neighbor, not to to be disgusting.