Abu al-Darda' (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Allah will forgive every sin, but He will not forgive the polytheist and the one who unjustly kills a believer."
Sadaqah said that Khalid said: Hani bin Kulthum bin Kinaz said that he heard Mahmud bin al-Rabi' narrate from Ubadah (may Allah be pleased with him) as marfu': "Whoever kills a believer and then rejoices at killing him, no act of worship of his, neither obligatory nor voluntary, is accepted." Likewise, in a narration of Ubadah (may Allah be pleased with him) there are also these words: "The believer continues to be righteous as long as he does not shed unlawful blood." Khalid said: I asked Yahya al-Ghassani what is meant by rejoicing at the killing. He said: The one who kills during fitnah (civil strife) and, having killed, thinks that he was in the right; Allah will never forgive him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15861