If Monika knew she was in a game and got rid of the others just to be with the player why didn't she just code her own ending that would make the most sense because that would make everyone happy and no one would hang themselves are stab themselves

Because then it wouldn't be a horror game, which is the whole point of DDLC - it's a game that seems like a normal dating sim, but it quickly takes a turn for the worse because of Monika's self-awareness. Also, I'm pretty sure she couldn't add in her own code; the most she could do is delete code or change it.