Do You Have To Kill EVERY SINGLE MONSTERS In Undertale Or Can You Leave Some Behind? [closed]

You don't have to kill anything, but the number of monsters killed will affect the ending. Generally speaking, you have three different endings:

Neutral ending

It occurs if you've killed at least one monster and it will change depending who lives and who dies. Asgore will die, you can escape the Underground and new person (or a dog) will be the new ruler. You will receive a phone call from Sans describing what is happening and Flowey will appear suggesting that you should try playing again without killing anyone

Pacifist ending

Don't kill anyone, at the end Asgore will be knocked out and a new boss - Asriel - will appear. After the fight you can console and forgive him and your friends will be able to go through the barrier.

Genocide ending

Achieved by killing EVERY creature. After killing Sans, you will meet Asgore and Flowey, the latter will beg you not to kill him. After their death, You will then come face to face with Chara, the original human who is also you in a weird way. Chara will praise you for your determination and power and will say the two of you can go on to destroy more worlds and erase this one. Regardless the game will end with a slash animation which implies the game itself is permanently damaged. This is somewhat true as your save files for future playthroughs will always be tainted after this ending.

Starting the game up again after this at any point will result in a blank screen with wind sounds. After 10 minutes Chara will appear asking if you think you are above consequences.

If you want to play again you will have to agree to give her your soul and it will alter your future play thoughs.