A lot of their jokes are really dark. On the episode introducing Flats the Flounder, as Spongebob runs screaming "he's going to kick my butt!" a crowd of people look at this one old guy and presumably beat him up to teach him a lesson. The old guy is saying "I love the young people", when it is the young people who are the ones beating him up. It happens twice in the episode.

Also another example, Sandy invents a machine that lets you hear what peanuts are saying. She then shows Spongebob her peanut blender. Spongebob is listening to the peanuts' screams.

TL;DR Spongebob is filled with dark humor, and it's fucking awesome.