Ahsanallahu ilaykum , if for example the usul of the Khawārij consists of 10/15 points and a person believes in 1 of them only( like speaking in public against the ruler). Why is he considered Khariji for only one thing?

He is considered to have followed the khawarij in this issue but u will find that they also fall into more violations may Allah protect us and you, therefore we can teach them and advise them with wisdom and stay away from their evil no need to call names.