Sahih Bukhari Volume 007, Book 069, Hadith Number 484.
Narrated By Abu Huraira : The Prophet said, “An adulterer, at the time he is committing illegal sexual intercourse is not a believer; and a person, at the time of drinking an alcoholic drink is not a believer; and a thief, at the time of stealing, is not a believer.” Ibn Shihab said: ‘Abdul Malik bin Abi Bakr bin ‘Abdur-Rahman bin Al- Harith bin Hisham told me that Abu Bakr used to narrate that narration to him on the authority of Abu Huraira. He used to add that Abu Bakr used to mention, besides the above cases, “And he who robs (takes illegally something by force) while the people are looking at him, is not a believer at the time he is robbing (taking).”
In other words: ‘ A murderer at the time of committing a murder is not a believer ‘.
Simple and straight forward and logical. We can say that according to this hadith the so-called Islamic militants / murderers can be looked at as non-believers.