However , they regard Ë AlÄ `` 's acceptance of arbitration at the Battle of Siffin against MuË Ä wiyya 's rebels as un-Islamic and as rendering him unfit for the Imamate , and they condemn Ë AlÄ `` for killing the early Kharijites of an-Nahr in the Battle of Nahrawan . However , they regard Ali 's acceptance of arbitration at the Battle of Siffin against Muawiya 's rebels as un-Islamic and as rendering him unfit for the Imamate , and they condemn Ali for killing the Muslims of an-Nahr in the Battle of Nahrawan .