What is the law/ruling (hukum) on someone who is a conspirator to apostacy (murtad) of muslims?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Disbelief is a very serious matter and it is one of the causes of the Wrath of Allaah, and which necessarily results in a life of eternity in Hellfire. Allaah says (what means): {Indeed, those who disbelieve and commit wrong [or injustice] — never will Allaah forgive them, nor will He guide them to a path. Except the path of Hell; they will abide therein forever. And that, for Allaah, is [always] easy.} [Quran 4:168-169]

Since this is the case with disbelief, then a person who endeavors to turn people into disbelievers has indeed committed a grave sin, and he will bear the sin of anyone who disbelieves because of him. Allaah says (what means): {And those who disbelieve say to those who believe, “Follow our way, and we will carry your sins.” But they will not carry anything of their sins. Indeed, they are liars. But they will surely carry their [own] burdens and [other] burdens along with their burdens, and they will surely be questioned on the Day of Resurrection about what they used to invent.} [Quran 29:12-13]

Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever calls others to guidance will have a reward like that of those who follow it, without that detracting from their reward in the slightest. And whoever calls others to misguidance will have a burden of sin like that of those who follow it, without it detracting from their burden in the slightest.” [Muslim]

Such a person must first be advised and threatened of the punishment of Allaah and  of the evil consequence of this act. If he is deterred, then this is what is desired; otherwise, his matter should be taken to the authorities so that they would deter him and stop him from doing so. It is the responsibility of those in charge of the Muslims to protect the belief of the community and protect it from any corruption. The Prophet, sallallaahu ‘alayhi wa sallam, and the rightly-guided Caliphs after him used to do so, as in the story of Thaat Anwaat as the Prophet, sallallaahu ‘alayhi wa sallam, denied their request to have a tree by which they would seek blessings as the polytheists did. Also, ‘Umar cut down the tree of allegiance (Shajarat Ar-Ridhwaan) when he saw people gathering at it, and he did so because he feared that their beliefs would be corrupted (by seeking blessing at that tree).

For more benefit, please refer to Fatwa 34046

Finally, it is worth pointing out that it is an obligation to teach the Muslims about their religion and their faith, and protect them from misconceptions which they might face. Calling to Allaah is an obligation which may be an individual or a communal obligation, according to the circumstances. Calling to Allaah is commanded as in the saying of Allaah (which means) {And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful.} [Quran 3:104]

Ibn Katheer said in his Tafseer regarding the interpretation of the above verse: “The meaning of this verse is that there should be a group from this Ummah who would carry out this duty, though this is an obligation on every single member of this nation according to his ability, as it is confirmed that Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever among you sees an evil, he should change it with his hand; if he is unable, he should change it with his tongue; and if he is unable, he should change it with his heart and that is the weakest of faith.” [Muslim] Another version reads: “…and there is not beyond this the weight of a mustard seed of faith.” [End of quote]

Allaah Knows best.