Minor shirk
Question

I have a colleague at work, and whilst I was talking to him he told us that he has an amulet that he got from some people who say that they are Shaykhs who treat people in the neighbourhood. He says that written inside it is Aayat al-Kursiy and he put it in a piece of leather and tied it and hung it on his chest. We had a discussion, myself, him and some of our colleagues. We advised him to burn the paper or take it to a trustworthy person to undo it and neutralize it, but he refused on the grounds that they are Shaykhs and that no one is allowed to see what is inside this thing, and it is beneficial. My reaction was to try to make him fear Allaah and I did not realize what I was saying; I said: “If you die with this thing on you, you will go to Hell.” To be honest, I regretted it after I said it, and I am afraid that Allaah may cancel out my good deeds, like the man who said to a sinner, “Allaah will not forgive you,” but Allaah said: “Who is the one who swore by Me that I will not forgive So and so? I have forgiven So and so and I have cancelled out your good deeds”. Now I am worried about this matter and I hope that you can advise me and tell me what I should do. May Allaah reward you with good.
Praise be to Allah.Firstly: 
It is not permissible to wear amulets even if they are from the Holy Quran, because of the general meaning of the words of the Prophet (peace and blessings of Allah be upon him): “Whoever wears an amulet, may Allah not fulfil his need, and whoever wears a sea-shell, may Allah not give him peace.”  Narrated by Ahmad (17440); classed as hasan by Shu’ayb al-Arna’oot in Tahqeeq al-Musnad. 
And he (peace and blessings of Allah be upon him) said: “Whoever wears an amulet has committed shirk.” This hadith was classed as saheeh by al-Albaani in Saheeh al-Jaami’. 
If amulets contain Quran, then there is a difference of scholarly opinion concerning them. The more correct view is that they are not allowed, because of the general meaning of the evidence, and so as to ward off the means (that may lead to shirk). And that is also because of the disrespect to which they are exposed in most cases, because the wearer sleeps wearing the amulet, and enters the toilet wearing it, and so on. 
It says in Fataawa al-Lajnah al-Daa’imah (1/212): The scholars are unanimously agreed that it is haraam to wear amulets if they contain something other than Quran, but they differed concerning those that contain Quran. Some of them allowed wearing them and some of them did not allow that. The view that it is not allowed is more correct, because of the general meaning of the ahaadeeth and so as to ward off the means (that may lead to shirk). End quote. 
This has been discussed in detail in the answer to question no. 10543. 
All of this applies if we accept that the amulet asked about here does not contain anything other than Aayat al-Kursiy or other words of Allah. But it may also include witchcraft and kufr. It is strange that that practitioners of witchcraft put the words of Allah with their kufr and falsehood, so as to conceal their evil and deceive the people. 
It seems from your colleague’s concern that no one should look inside the amulet and his refusal when you asked him for the amulet so that you could see what was inside it, that your colleague is either ignorant of what is really inside the amulet or he knows that what is inside is not only the words of Allah. 
Based on that, you have to advise this colleague of yours and warn him against wearing anything the contents of which he does not know, and wearing amulets altogether. 
His saying that no one is allowed to see what is inside it is not correct, rather this is one of the lies of the practitioners of witchcraft and charlatans, and is a way in which they scare the people so that they will not see what is inside the amulets of the names of devils or writing that has no meaning. 
Secondly: 
As for your saying “By Allah, if you die with this thing on you, you will go to Hell”, if what you meant is that the one who does that deserves to go to Hell, based on what you have heard about the Prophet (peace and blessings of Allah be upon him) forbidding that, and his warning of Hell to the one who did that, whilst also knowing that we cannot know whether any specific person will go to Paradise or Hell, except those who are mentioned in the Wahy (Revelation), and that believers in Tawheed who commit sin are subject to the will of Allah, and if He wills He will punish them and if He wills He will forgive them – if that is what you meant, then there is no sin on you in sha Allah. 
Similarly, if your intention was to warn him of the punishment of those who take such matters lightly and persist in doing them, there is no sin on you for that, in sha Allah. 
In Musnad al-Imam Ahmad (19498) it is narrated that al-Hasan said: ‘Imraan ibn Husayn told me that the Prophet (peace and blessings of Allah be upon him) saw a ring, I think he said of brass, on a man’s upper arm. He said: “Woe to you, what is this?” He said: It is for pain in the shoulder and hand. He said: “It will only increase you in weakness. Take it off, for if you die with it on you, you will never succeed.” [Classed as da’eef by al-Albaani in al-Da’eefah (1029)]. 
In Sunan Abi Dawood (4669) it is narrated that Ibn al-Daylami said: I went to Ubayy ibn Ka’b and said to him: I am rather confused about al-qadar (the divine decree); tell me of something by means of which Allah may take away that which is in my heart. He said: If Allah punished the inhabitants of His heavens and the inhabitants of His earth, He would punish them and would not be unjust towards them. If He were to bestow mercy on them, His mercy would be better for them than their good deeds. If you were to spend the equivalent of Uhud in gold for the sake of Allah, Allah would not accept it from you unless you believed in al-qadar and knew that whatever befalls you could not have missed you and whatever missed you could not have befallen you. If you die believing something other than this, you will go to Hell. He said: Then I went to ‘Abd-Allah ibn Mas’ood and he said something similar. Then I went to Hudhayfah ibn al-Yamaan and he said something similar. Then I went to Zayd ibn Thaabit and he narrated something similar to me from the Prophet (peace and blessings of Allah be upon him). 
Classed as saheeh by al-Albaani in Saheeh al-Jaami’ (5344). 
Al-Qaari said concerning the words “you will go to Hell”: this is a warning and a threat! End quote from Tuhfat al-Ahwadhi. 
But if what you meant was that this specific person is one of the people of Hell because of this action of his, this is a mistake on your part. We cannot testify that any specific person will be in Paradise or in Hell, unless there is a text that testifies to that. 
Note: Wearing an amulet may be minor shirk or it may be major shirk, depending on the situation of the one who uses it. If he believes that it can bring benefit and cause harm in and of itself, then this is major shirk. If he believes that it is a means, then this is minor shirk, because he is regarding something that is not a means as a means. If a person dies in a state of minor shirk without having repented, is he subject to the will of Allah, or will he not be forgiven, as in the case of major shirk? There are two scholarly points of view: 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Do the words of Allah, “Verily, Allah forgives not that partners should be set up with Him (in worship)” [al-Nisa’ 4:48] include minor shirk? 
He replied: The scholars differed concerning that. Some of them said that it includes all kinds of shirk, even minor shirk such as swearing by something other than Allah; Allah will not forgive him. But with regard to major sins, such as drinking alcohol and zina, they are subject to the will of Allah; if Allah wills He will forgive them and if He wills He will punish them. 
Shaykh al-Islam [Ibn Taymiyah] said different things. On one occasion he said: Allah does not forgive shirk even if it is minor. And on another occasion he said: That which Allah does not forgive is major shirk. 
Whatever the case, we must beware of shirk altogether, because the general meaning may include minor shirk too, because He says “Verily, Allah forgives not that partners should be set up with Him” meaning any partner, therefore it suggests any kind of shirk. End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (2/203). 
As for what you have to do, you have to repent to Allah from your taking such wording lightly, and not do that again in the future.  
And Allah knows best.