Because that's the tragedy of Fern as a character.
What you have to keep in mind is that Fern is literally Finn, just at a displaced moment in time from the creation of the Finn sword. Then you pair this with the Grass Demon, who has infected this version of Finn, depressing him and making him think violent thoughts. By the time he actually manifests as Fern, that version of Finn has been through quite a lot.
The real kicker is that because Fern is an actual Finn, he has all the same memories, connections, and experiences as the main Finn. Despite this, he is treated completely different by everyone he ever knew, through no fault of his own. Even Jake treats Fern differently than Finn. Could you imagine if your own brother, who you've spend your whole life with, one day treated you like a completely different person?
In Ferns mind, he and Finn should be exactly the same, because they are. He just wants to eliminate what he sees as an exact copy of himself, that has ostracized him from everyone else and seemingly hogging his identity.