Watch the episode "Memory of a Memory", it tells about Marceline's childhood, and important memories. If you pay attention to the first memory, of Marceline only about 5-7 years old, she's living in some kind of place with tanks, fires, homes, etc. What time would that be from? When the humans still existed?
The land, she knew, would never be the same. She knew that all of this destruction, this hate, would lead to nothing. What she didn't know, was why it had to come to this. Couldn't it be safe, one more time? Apparently not, according to the soldiers, the leaders, even her father. Her mother was the only one she knows, who cries every night mourning what was lost, what would never be again.
It was the middle of the day. Just like any other, it was worse than the previous. The smoke from the bombs still climbs into the endless sky, now a horrid purple color from its once pretty, peaceful blue. All around, the trees' leaves dimmed, shrivel, and gently float to the ground. The air had the usual stench of fumes, threatening to kill anyone who took too deep a breath. All she was thankful for was the fact that this world, what had become of it at least, could not touch her. Unless, she let it go to her head.
"Oh Hambo, you're my only friend," she sighs, hugging her rag bear close as fiddles with his new eye. The smell of mold coming from his fur almost makes her gag. "Hambo, you smell!" She laughs, "C'mon, lets go give you a bath," She standing from her place on the ground on the road with a broken tank. Completely ignoring the war machine, she jumps over it easily, skipping towards the stream.
Soon enough, she can hear the water flowing down its path, not nearly good for drinking, more for starting fires. It was polluted far beyond from the wars.
Slowly, she dunks Hambo into the water, ridding him of his earthly smell. Once entirely soaked, she hunts for wild flowers to rub him with. She knew that it was near hopeless to find a pretty one to give her mother, let alone one that smells nice. But, she finds decent red flowers that she sucks the color from, then rubs the pollen evenly over his body. After, his scent is almost unnoticeable.
"Alright Hambo, let's go exploring in the ruins for something useful. Maybe we'll find a brother for you!" She races past the stream, easily jumping over it with one leap. The dead or near dying trees slowly disperse, and end when she comes to the ruins of what was once called New York City. She was always amazed with the enormous buildings, that scraped the purple sky, bit by bit turning to rubble, collapsing onto itself. Nearly every month, one more building was a pile of rocks and rusted metal.
Quietly walking through the deserted streets, littered with contraptions called Cars, and old weapons from the battles, only about twenty years ago.
Her extremely slow vampire aging process was noticeable. She remembers, before the war, that humans her age were in adulthood. Old friends out grew her, but then she found Hambo. She knew he would never ever leave her, they loved each other too much, and he wouldn't "Out Grow" her.
Unlike those humans, he didn't go with that other girl into the ground to escape their own destruction up here. All she could think at that moment, was how stupid they were to run from their mistakes. Then, over years, she began to pity their children. By this time, they must have forgotten what a bird sounds like, what wind feels like, what the sun is. That girl that gave her Hambo, must have forgotten who she was, and Hambo himself.
All she could do, was pity them.
"Huh, would you look at that," she holds the bear in front of her, to show him an ancient picture on a wall of a bowl of fruit. "the silly things they painted." She moves on, totally oblivious to what this place once was. What it meant to people.
Who lived here.
What, in 1,000 years, this land would be called. The Land of Ooo.
Then again, she wouldn't tell a soul about the old days, the war, the hate, the destruction. If she did, she knew it would happen again. The peaceful land of Ooo, the princesses, the monsters, not Finn and Jake. She swore, to never tell them about that time, when the humans declared war on themselves, killed each other, ruining their lives, their chances of survival… including her's.
That's why, when she met Finn and found out he was a human, not some goblin, she kicked him out of his house. At least, she could do that to the humans, for ending the world.
For ending it all.