Spilling the Haraam blood and Takfeer of Muslims
… Spilling the Haraam blood and making Takfeer of Muslims are of the great sins with Allah (swt) and His Messenger (saw): As for shedding the Haraam blood then At-Tirmidhi narrated in his Sunan from Abdullah Bin ‘Amr that the Nabi (prophet) (saw) said: «لَزَوَالُ الدُّنْيَا أَهْوَنُ عَلَى اللَّهِ مِنْ قَتْلِ رَجُلٍ مُسْلِمٍ» “Verily, the perishing of the Dunyaa (world) is less upon Allah than the killing a Muslim man”…An-Nisaa’i narrated from Abdullah Bin ‘Amr from the Nabi (Prophet) (saw) that he said: «لَزَوَالُ الدُّنْيَا أَهْوَنُ عِنْدَ اللَّهِ مِنْ قَتْلِ رَجُلٍ مُسْلِمٍ» “Verily, the perishing of the Dunyaa (world) is less with Allah than the killing of a Muslim man”…
As for making Takfeer of the Muslim, Bukhari related in his Saheeh from Abdullah Ibn ‘Umar (ra): That the Messenger of Allah (saw) said: «أَيُّمَا رَجُلٍ قَالَ لِأَخِيهِ يَا كَافِرُ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا» “Any man that says to his brother O Kaafir, then that (description) has befallen one of them”… And Muslim narrated from Ibn ‘Umar the wording (Lafzh) that the Prophet (saw) said: «إِذَا كَفَّرَ الرَّجُلُ أَخَاهُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا» “If the man makes Takfeer of his brother then it (the description) has befallen one of the two”… And it was mentioned in Al-Adab Al-Mufrad from Abdullah Ibn ‘Umar that the Messenger of Allah (saw) said: «إِذَا قَالَ لِلْآخَرِ: كَافِرٌ، فَقَدْ كَفَرَ أَحَدُهُمَا، إِنْ كَانَ الَّذِي قَالَ لَهُ كَافِرًا فَقَدْ صَدَقَ، وَإِنْ لَمْ يَكُنْ كَمَا قَالَ لَهُ فَقَدْ بَاءَ الَّذِي قَالَ لَهُ بِالْكُفْرِ» “If it is said to another: Kafir. Then one of them has disbelieved. If the one whom it is said to is a Kafir then he has been truthful and if he is not as he has said, then what he said to him will return back to him in terms of disbelief”.
As such, the shedding of the Haraam blood of the Muslim is an extremely major sin and similarly making Takfeer of a Muslim is an extremely major sin.