When two muslims engage in combat
Abu Bakrah Ath-Thaqafi (radi Allahu anhu) reported that the Messenger of Allah (sal Allahu alaihi wa sallam) said: “When two Muslims are engaged in a combat against each other with their sword’s and one is killed, both are doomed to Hell”. I said, “O Messenger of Allah! As to the one who kills, it is understandable, but why the slain one?” He (sal Allahu alaihi wa sallam) replied: “He was eager to kill his opponent”
[Al-Bukhari and Muslim]
This Hadith leads to the conclusion that one would be punished for such a sinful intention for which he has made a firm determination, and for the commitment of which he has adopted necessary measures, even if he does not succeed in committing it because of certain obstruction.
Thus determination is different from a suggestion of the devil. The latter is excusable while one is accountable for his determination. However, what is mentioned in the Hadith will happen when Muslims fight among themselves for worldly honour and prejudices, and no religious issue would be the cause of their conflict, because in the latter case, it is possible that both might be depending on their own Ijtihad (exercise of opinion) for which they may be excused.
And Allah Knows Best!