Is it Kufr to fight or kill a Muslim?

[Sahih Muslim 1676 a](https://sunnah.com/muslim:1676a) says > It is not permissible to take the life of a Muslim who bears testimony (to the fact that there is no god but Allah, and I am the Messenger of Allah, but in one of the three cases: the married adulterer, a life for life, and the deserter of his Din (Islam), abandoning the community. So in only 3 cases a Muslim can be killed. For adultery, for apostasy and for murder. However, in the case of murdering a non Muslim, a Muslim can’t be killed as said by the Prophet PBUH {source: [Sunan Ibn Majah 2660](https://sunnah.com/ibnmajah:2660)} Which leaves us with the last case: A Muslim murdering a Muslim. But in that case, he would be committing kufr {source: [Sahih Al-Bukhari 48](https://sunnah.com/bukhari:48)} . That means he would be committing apostasy. So in that case wouldn’t murder and apostasy come under the same line? Then why are they mentioned differently?