Whoever supports non-Muslims is regarded an apostate then follows the permissibility of killing him. Consequently, they declared the whole Muslims as disbelievers and declared the permissibility of killing Muslims and unlawfully seizing their properties.
Those who support non-Muslims are apostates. So, it is permissible to kill them. Consequently, they declared the whole Muslims as disbelievers and deemed violating Muslims' lives and honors as lawful
Supporting non-Muslims, even if they are from our enemies, is not an act of disbelief. However, it is a heinous crime that necessitates punishment by the ruler to deter the traitor, but he remains a Muslim. There is a hadith reported by al-Bukhari supporting this view, which states that Ali (May Allah be Pleased with him) said: Allah's Messenger (PBUH) sent me along with al-Zubair and Al-Miqdad and said, "Proceed till you reach a place called Raudat-Khakh where there is a lady travelling in a howdah on a camel. She has a letter. Take the letter from her." So we set out, and our horses ran at full pace till we reached Raudat Khakh, and behold, we saw the lady and said (to her), "Take out the letter!" She said, "I have no letter with me." We said, "Either you take out the letter or we will strip you of your clothes." So she took the letter out of her hair braid. We brought the letter to the Prophet and behold, it was addressed by Hatib ibn Abi Balta'a to some pagans at Mecca, informing them of some of the affairs of the Prophet. The Prophet said, "What is this, O Hatib?" Hatib replied, "Do not be hasty with me, O Allah's Messenger! I am an Ansari man and do not belong to them (Quraish infidels) while the emigrants who were with you had their relatives who used to protect their families and properties at Mecca. So, to compensate for not having blood relation with them.' I intended to do them some favor so that they might protect my relatives (at Mecca), and I did not do this out of disbelief or an inclination to desert my religion." The Prophet then said (to his companions), "He (Hatib) has told you the truth." `Umar said, "O Allah's Apostle! Allow me to chop his head off?" The Prophet said, "He is one of those who witnessed (fought in) the Battle of Badr, and what do you know, perhaps Allah looked upon the people of Badr (Badr warriors) and said, 'Do what you want as I have forgiven you). The Prophet neither (PBUH) killed Hatib ibn Abi Balta'a nor declared him as disbeliever.
However, such crime is a heinous one that entails a punishment which is decided by the ruler and may even reach execution.