Chara isn't a demon. Sure they may appear at the end of the genocide run, but I have a reason for why they do. Chara want's you to erase the world to find another Undertale version code, and send you there to restart your journey for a pacifist run. If you don't choose to erase the world, Chara will be depressed a bit because that's because they'll never be able to see their brother again. As we all know, Flowey is Asriel, and killing Flowey just hurts Chara even more and makes them hate humanity even more.
During the pacifist run, when going to True Lab, you can find memory tapes of the Dreemurr family, and Chara & Asriel's plan was to set everyone free. Asriel even says: "Remember when the recipe said 2 cups of butter but we used buttercups instead? Yeah, it made dad pretty sick.. made mom cry.." Chara wasn't intending to hurt anyone with their new life, Asriel brought HOPE to Chara for the 1st time in their life.
When Chara died, Asriel absorbed their soul, regretting what he's done later, in god form, they went to the surface as one, Asriel went to Chara's home, and with Chara's own power, they wanted extermination for everyone they knew because of how bad they treated Chara as a person. Asriel held back all of the true power with his own willpower, and as humans wished their death, he smiled with no regrets, knowing he's done all he could. He died and became Flowey.
How Asriel became Flowey: Either Gaster or Alphys put Flower/Buttercup DNA into his system when he was still alive, which is how he became Flowey in the first place. Flowey wanted it to be "Kill or be Killed" because that's all he thought the world was after the humans killed him, he didn't want peace, he just wanted a world to himself so he could show everyone on the surface that he was back and ready to kill everyone so HE could have a kingdom to himself, a full world(Frisk prevented that).
Cannon Chara loves chocolate as everyone knows, BUT everyone blamed Chara for murder because they appear at the end of the genocide run, heck even animators do it to. But the Player(us/the person who plays the game) chooses to kill everyone just for "an ending" since theirs's 3. the only people aware of this, is Chara, Sans, Flowey & the Player. Sans does this by breaking the 4th wall when he made his puns back in Snowdin forest when he would turn to the camera. Chara breaks it at the end of genocide. Flowey breaks the 4th wall in general, knowing about the Save files.