This just a fight between which Islamic version you have.....Something ridiculous and not right as killing another Muslim means being cursed by God and went to Hell. If I want to fight another Muslim in a war due to just different interpretation of Islam I would also have doubt in my heart to kill them........It is some group who dont have any doubt to kill another Muslim that should be seen as less Islamic.....
Quran
Surah Ar Rum verse 32
(Of those associators) who are divided in their Religion and became sects (fractions), every fraction rejoicing in what is with it.
Surah Al AnAm verse 159
Surely they who divided their religion and became sects. You have no concern with them. Their affair is only with Allah, then He will inform them of what they used to do.
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This is when Muslim has different opinion and how to solve it
Surah As Syura verse 38
And they respond to their Lord and keep up the Prayer. And they gather amongst each other to conduct their affairs by mutual consultation. And they spend out of what We have given them as sustenance.
The punisment of killing another Muslim
An Nisa verse 93
And whoever kills a believer intentionally, then his punishment is Hell; he shall abide in it forever; Allah has sent His Wrath on him and cursed him and He has prepared for him a great torment.
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This is when we see two Muslim has conflict
Surah Al Hujurat verse 10
"The believers are only brothers and sisters. Therefore make reconciliation between your brothers and sisters, and have piety towards Allah. It is hoped that you may be given Mercy."