Not all episodes have happy endings or conflict resolution. But if the main characters are mutated, injured or killed at the end of a episode, they always return intact for the next episode.
The "Halloween Spectacular of Spooky Doom" has some particularly dark and nightmare-ish imagery that children may find frightening.
Sometimes characters will be turned inside out revealing their internal organs; this is done in an intentionally gross way, with large pulsing veins, a dark color pallet, bodily fluids leaking out etc. young kids may find this to be disgusting or unsettling. ("Halloween Spectacular of Spooky Doom"; "Dark Harvest"; "Bloaty's Pizza Hog")
There are many disgusting scenes and stories that may make some people, regardless of age, uncomfortable. For reference, the episodes: "Germs", "Lice", "Bolognius Maximus", "Rise of the Zitboy" and "Dark Harvest" have some of the most disgusting imagery/concepts in the series.
Invader ZIM is a very dark, cruel, violent, sarcastic and satirical show, especially for one on a children's network. The content varies from episode to episode, but some children may find this show to be too frightening, disgusting or disturbing. The series has an intentionally dark and unfriendly atmosphere, several episodes play out like a horror movie and there is never any genuine affection or warmth between characters.
Dark Harvest involves around Zim harvesting organs from children. In real life, a 17-years old teenager named Scott Dyleski murdered his neighbor because the episode made him wonder what is inside a human body. The people who defended Dyleski said that these comments were made in jest, but they also said that Dyleski is innocent, which is clearly false. Bottom line - an episode of the series might have inspired a real-life murder.
Some of the experiments performed on the children in the episode "Lice" vaguely resemble Nazi human experimentation. In fact, the episode "Lice" was originally going to be called "Lice Nazi" but Nickelodeon denied this title.
Some kids may find Gaz's cynical and demonic nature to be unsettling and/or frightening, particularly in the episodes "Gaz, Taster of Pork" and "Game Slave 2".
Many of the background characters and side characters have large pulsing boils or warts, hideous deformities, giant scars on their faces and/or other disgusting and unappealing features. Very young kids may find these character's appearances to be unsettling.
There are many episodes with plots centered around harsh bullying and/or revenge. This includes, but is not limited to the episodes: "Bolognius Maximus", "Game Slave 2", "Gaz, Taster of Pork", "The Wettening", "Bad, Bad Rubber Piggy", "Megadoomer", "Tak, the Hideous New Girl", and "A Room With a Moose". In all of these instances, the problem, if solved, is solved with cruelty and/or violence. Some viewers may find this upsetting.
In the episode, "Zim Eats Waffles", Zim briefly probes and experiments on the brain of a child. In the episode, "Bloaty's Pizza Hog" Zim performs a vivisection on Dib by taking all of his organs outside of his body. This action is done off-screen, but a pile of his organs outside of his body is briefly seen on-screen and although it's not bloody, it's still pretty gross looking.
In the episode "Bad, Bad Rubber Piggy", Zim physically harms Dib multiple times in rather harsh (although very exaggerated) ways which causes Dib to suffer a variety of lasting injuries, each one worse than the last. Zim eventually succeeds at killing Dib off (which was his plan), but Dib quickly comes back to life.
Almost all of the music in the series is suspenseful and/or horrific sounding.
There's some scenes that are meant to shock viewers and some scenes that are a little bit graphic, but these scenes are usually brief.
In one episode, Dib is seen doodling a picture of Zim strapped to a table and being vivisected.
This show is extremely dark and sometimes depending on the person it can be really disturbing, there is plenty of stuff that could make you stop watching if you are easily sickened by gross content, and comedy at its darkest at times, also if you don't like organ stuff like if are slightly disturbed or grossed out by organ stuff than just don't watch it.
There are pretty much no positive lessons in this show whatsoever, and almost everything is resolved with violence or manipulation.
A robot clone of Dib gets strangled and his eye pops out.
There is an episode where Zim steals other childrens organs and puts them inside him, the episode is very disturbing.
In the episode "Bestest Friend" a kid has his eyes ripped out by two claws. Only a shadow of this is shown and his eyes are replaced with robotic ones but we can hear him screaming in terror as horrific music plays. Later in the episode, the same kid is attacked by a squirrel and falls off of a tall building as the screen turns red and fades to black.
In "Game Slave 2" a kid is inside of an elevator that snaps and falls down fifty stories; he is later shown to have survived.
There are multiple scenes in which very minor characters are crushed, swallowed whole, blown up etc. and it is NOT shown if they survived.
The bulk of the episodes "Mortos der Soul Stealer", "Dark Harvest", "Halloween Spectacular of Spooky Doom" and "Gaz, Taster of Pork" are based heavily on dark and horror-filled themes amongst the sillier nature of their plots.
The uncut edition of the Christmas special has an on-screen suicide joke, in which debris falls on a crowd of people and one man holds out his arms, smiling and happy to be crushed to death. The impact was one of the things cut out in the cut edition. (It's worth noting that the uncut edition of the Christmas special can be difficult to find.)