6 was afraid of 7 because 7 ate 9, but why was 10 scared?
Because 10 was in 9/11.
Listen up, buttercup. The joke plays on the phrase "7 ate 9" and the number 10 being "in" (as in, part of) the 9/11 attacks. It's dark humor at its finest, which is probably the only thing you'll ever find fine, given your intellectual prowess.