In an episode where these little people are terrorising Finn and Jake the both of them decide to use ketchup as a fake effect of blood when they kill them.
Even most of the violence present in this cartoon is mild and it's mostly towards fantasy characters (think of punches, kicks, swordfights... and the consequent bruises and scratches), in some cases this can be a little more graphic and even include red blood and gore, but that's rare.
The series mainly takes place in a post-apocalyptic world. The opening has a dismembered arm inside a hollow tree. Very fast, can't be seen without pausing. Fans think the arm belongs to Shoko.
In "The Enchiridion", Finn kills a monster shaped as a heart, red blood sprouts all over. Very brief and the blood might have not stained anything.
Some decapitation of small parts (noses and hands), or limb removal, mauling (i.e. arms chopped off) throughout the series, but no blood, only flesh wounds.
Several fight scenes are played out for humor. For example, there is a character named Ash, who everyone hates, since he is a douche. Marceline kicks his crotch and Finn starts beating him up with her. Finally Jake stomps him with his giant foot. Even the Ice King (Simon Petrikov) in one episode punches him with no good reason.
In "Who Would Win", Finn and Jake start beating up each other, because they are proving themselves who is more resilient. Both of them have lots of nasty looking bruises and cuts.
In "Freak City", the Magic Man turns a bird inside out making it show the birds guts, flesh and everything else.
Some scatological humor, including spitting and visible green vomit in some episodes, when the characters are disgusted. For example, Jake explicitly vomits into a jacuzzi in the episode "Jake the Dog". Farting is more frequent.
In "A Glitch is a Glitch", a pale skinned, creepy looking woman in a TV eats her own hair. She gags a lot, and is all the time ready to vomit. Jake and Finn are absolutely disgusted.
In "Princess Potluck", Finn has a rash on his leg in one episode. Some kind of disgusting pink liquid/puss drips from it.
Skeletons are seen in some episodes also, walking skeleton get smashed. Some characters practice necromancy.
In "Princess Monster Wife", the Ice King stole parts of other princesses bodies and made a monster out of them. Princess Bubblegum got half her face torn-off to set an example.
In "Daddy's Little Monster", a demon gets half his body torn off by a laser. All his insides show.
In "Return of the Nightosphere", a demon starts vomiting bananas and another one gets his head turned into abdominal muscles.
When Hunson Abadeer (The lord of evil), appears from The Nightosphere (Hell), he reconstructs himself to the real world in a gory manner. All his organs, blood and skin stick together, and create him in a brief (half a second) scene. This happens in "It Came from the Nightosphere" and in "Marcy & Hunson".
In "All Your Fault", is seen that Castle Lemongrab has rooms filled with guts, a room with a giant heart and a room with a monster that peels it's skin off to reveal it's skull.
In "Escape from the Citadel", The Lich has skin peeling off his ancient, rotten flesh and what seems to be veins hanging from his arms. He looks very disturbing indeed.
In "Mortal Folly", Finn presumably "kills" the Lich by shoving a sweater inside his (the Lich's) eye socket, and proceeds to pull through the other socket so that the sweater comes out between the sockets. And it does, breaking the Lich's skull.
In "Goliad", the homonymous character squashes an innocent bee.
In "The Pit", a demon sucks up almost all of Jake's blood. The blood is shown. A sword is made out of a demon's blood.
In "Mystery Dungeon", a rat appears on scene and grabs a pie, then Lemongrab punches it and kills it and he starts to eat that pie from its mouth.
In "The Great Bird Man", we see the eyes (with their roots) from a man, and they're put on and extracted at some points of an episode.
Some scenes from the miniseries "Stakes" include fingers briefly burning from the sun, a neck being pierced, a stake coming from the skin, animals dying and a character sucking blood from a horse.
In the episode titled "The Jiggler" a creature that Finn and Jake take home becomes violently ill and spews what appears to be blood or vomit out of many orifices of its body. In an effort to stop the leakage, Finn and Jake plug up the holes, only to have the creature expand and eventually burst. Jake and Finn are covered in twisted strands of the creature's body as Jake remarks "Our pet exploded".
References to violence and situations of threat are also frequent; this include "slay you and munch your eyeballs", "rip your throat", "unzip your skin", "rip your heart", "my head will be decapitated", "we'll suck their guts out through their brains", "pull that arm", etc. None of this finally occurs.