Cursing a believer or calling accusing him of kufr (disbelief) is akin to murder

who was one of the companions of the tree (who gave the pledge of allegiance to the Prophet ﷺ underneath the tree at Ḥudaybiyyah) that the Messenger of Allah ﷺ said, "Whoever swears by a religion other than Islam then he is really so. A child of Adam is not bound to fulfill a vow to Allah about a thing which he does not possess."

"If someone commits suicide with anything in this world, he will be tortured with it on the Day of Resurrection. If somebody curses a believer then his sin will be as if he killed him. And whoever accuses a believer of kufr (disbelief) then it is as if he killed him."  

وَكَانَ مِنْ أَصْحَابِ الشَّجَرَةِ حَدَّثَهُ أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «مَنْ حَلَفَ عَلَى مِلَّةٍ غَيْرِ الإِسْلاَمِ فَهْوَ كَمَا قَالَ؛ وَلَيْسَ عَلَى ابْنِ آدَمَ نَذْرٌ فِيمَا لاَ يَمْلِكُ؛»

«وَمَنْ قَتَلَ نَفْسَهُ بِشَىْءٍ فِي الدُّنْيَا عُذِّبَ بِهِ يَوْمَ الْقِيَامَةِ؛ وَمَنْ لَعَنَ مُؤْمِنًا فَهْوَ كَقَتْلِهِ؛ وَمَنْ قَذَفَ مُؤْمِنًا بِكُفْرٍ فَهْوَ كَقَتْلِهِ۔»  

The Prophet ﷺ said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him."  

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ حَلَفَ بِمِلَّةٍ غَيْرِ الإِسْلاَمِ كَاذِبًا فَهْوَ كَمَا قَالَ وَمَنْ قَتَلَ نَفْسَهُ بِشَىْءٍ عُذِّبَ بِهِ فِي نَارِ جَهَنَّمَ وَلَعْنُ الْمُؤْمِنِ كَقَتْلِهِ وَمَنْ رَمَى مُؤْمِنًا بِكُفْرٍ فَهْوَ كَقَتْلِهِ  

It does not seem proper for a Siddiq that he should be an invoker of curse.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ يَنْبَغِي لِصِدِّيقٍ أَنْ يَكُونَ لَعَّانًا  

I heard Messenger of Allah ﷺ as saying: The invoker of curse would neither be witness nor intercessor on the Day of Resurrection.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ إِنَّ اللَّعَّانِينَ لاَ يَكُونُونَ شُهَدَاءَ وَلاَ شُفَعَاءَ يَوْمَ الْقِيَامَةِ  

The Prophet ﷺ said: Do not invoke Allah's curse, Allah's anger, or Hell.  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ تَلاَعَنُوا بِلَعْنَةِ اللَّهِ وَلاَ بِغَضَبِ اللَّهِ وَلاَ بِالنَّارِ  

"The believer does not insult the honor of others, nor curse, nor commit Fahishah, nor is he foul."  

قَالَ رَسُولُ اللَّهِ ﷺ لَيْسَ الْمُؤْمِنُ بِالطَّعَّانِ وَلاَ اللَّعَّانِ وَلاَ الْفَاحِشِ وَلاَ الْبَذِيءِ  

when a man cures anything, the curse goes up to heaven and the gates of heaven are locked against it. Then it comes down to the earth and its gates are locked against it. Then it goes right and left, and if it finds no place of entrance it returns to the thing which was cursed, and if it finds no place of entrance it returns to the thing which was cursed, and if it deserves what was said (it enters it), otherwise it returns to the one who uttered it. Abu Dawud said : Marwan b. Muhammad said: He is Rabah b. al-Walid who heard from him (nimran). He (Marwan b. Muhammad) said: Yahya b. Hussain was confused in it.  

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ الْعَبْدَ إِذَا لَعَنَ شَيْئًا صَعِدَتِ اللَّعْنَةُ إِلَى السَّمَاءِ فَتُغْلَقُ أَبْوَابُ السَّمَاءِ دُونَهَا ثُمَّ تَهِبْطُ إِلَى الأَرْضِ فَتُغْلَقُ أَبْوَابُهَا دُونَهَا ثُمَّ تَأْخُذُ يَمِينًا وَشِمَالاً فَإِذَا لَمْ تَجِدْ مَسَاغًا رَجَعَتْ إِلَى الَّذِي لُعِنَ فَإِنْ كَانَ لِذَلِكَ أَهْلاً وَإِلاَّ رَجَعَتْ إِلَى قَائِلِهَا  

We were with Messenger of Allah ﷺ in some of his journeys and there was a woman from the Ansar riding a she-camel that it shied and she invoked curse upon that. Messenger of Allah ﷺ heard it and said: Unload that and set it free for it is accursed. 'Imran said: I still perceive that (dromedary) walking amongst people and none taking any notice of that.  

بَيْنَمَا رَسُولُ اللَّهِ ﷺ فِي بَعْضِ أَسْفَارِهِ وَامْرَأَةٌ مِنَ الأَنْصَارِ عَلَى نَاقَةٍ فَضَجِرَتْ فَلَعَنَتْهَا فَسَمِعَ ذَلِكَ رَسُولُ اللَّهِ ﷺ فَقَالَ خُذُوا مَا عَلَيْهَا وَدَعُوهَا فَإِنَّهَا مَلْعُونَةٌ قَالَ عِمْرَانُ فَكَأَنِّي أَرَاهَا الآنَ تَمْشِي فِي النَّاسِ مَا يَعْرِضُ لَهَا أَحَدٌ