Hadith: Verily, what I fear for you is a man who recites the Qur’an until its brightness appears on him and he becomes a support of Islam. (This continues) until this man changes his state to what Allah wills. He detaches himself from it and throws it behind his back. Then he starts attacking his neighbor with the sword, accusing him of polytheism

عن حذيفة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «إنَّ ما أَتَخوَّفُ عليكم رجلٌ قَرَأ القرآنَ حتى إذا رُئِيَتْ بَهْجتُه عليه، وكان رِدْئًا للإِسلام، غَيَّرَه إلى ما شاء الله، فانْسَلَخَ مِنْه ونَبَذَه وراءَ ظَهْرِه، وسَعَى على جاره بالسَّيف، ورمَاه بالشِّرك»، قال: قلتُ: يا نبيَّ الله، أيُّهما أوْلى بالشِّرك، المَرْمِي أم الرَّامي؟ قال: «بل الرَّامي».
[حسن] - [رواه ابن حبان]

Hudhayfah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Verily, what I fear for you is a man who recites the Qur’an until its brightness appears on him and he becomes a support of Islam. (This continues) until this man changes his state to what Allah wills. He detaches himself from it and throws it behind his back. Then he starts attacking his neighbor with the sword, accusing him of polytheism." I asked: "O Prophet, which of the two deserves more to be described as a polytheist? The accused or the accuser?" He replied: "The accuser."
[Good hadith] - [Narrated by Ibn Hebban]

Explanation

The thing the Prophet (may Allah's peace and blessings be upon him) feared most for his Ummah is a man who read the Qur’an so much that the people noticed its brightness, beauty, and good influence appearing on him. He was also a supporter of Islam and defender of the Muslims until he changes his attitude, gives up Islam, abandons the Qur’an, and kills his neighbor after accusing him of polytheism. The Prophet (may Allah's peace and blessings be upon him) was asked: which of the two will be deserving of such a description: the man who killed his neighbor and accused him of polytheism, or the murdered neighbor? The Prophet (may Allah's peace and blessings be upon him) replied that the one who deserves this description is the man who killed his neighbor and accused him of polytheism.