Assalaamu alaykum. I was wondering; if someone loves sin in their heart and hates Allaah, like a serial killer, but he nevertheless believes in Him and agrees with everything that Allaah says while not caring about that, so he believes that halal is halal and haram is haram but does not apply that, is he a disbeliever or not?

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

A Muslim does not go out of the fold of Islam just because of killing, even if he has done this heinous act many times, unless he considers killing to be lawful, in which case he goes out of the fold of Islam because of making this haraam (unlawful act) halaal (lawful).

Allah says (what means): {And if two factions among the believers should fight, then make settlement between the two. But if one of them oppresses the other, then fight against the one that oppresses until it returns to the ordinance of Allah. And if it returns, then make settlement between them in justice and act justly. Indeed, Allah loves those who act justly. The believers are but brothers, so make settlement between your brothers. And fear Allah that you may receive mercy.} [Quran 49: 9-10]

Ibn Taymiyyah said regarding the above verses, “He called them 'believers' and considered them 'brothers' though there is fighting and oppression between them.”

The Prophet said, “If two Muslims meet with their swords [each one willing to kill the other]; the killer and the killed are in Hellfire...” [Al-Bukhari and Muslim]

Ibn Battaal in his commentary on Saheeh Al-Bukhari, said, “He called them Muslims even though one of them kills the other. The belief of Ahlus-Sunnah wal-Jamaa’ah is that Allah’s threat to the disobedient believers is left to His will; if He wills, He punishes them, and if He wills, He pardons them.”

The fact that a believer kills does not necessarily mean that he hates Allah, the Lord of the worlds, and the one whom you described as believing in Allah and agreeing with what Allah says cannot be someone who hates Allah. So he still remains a believer in general, but at the same time, he is dissolute (Faasiq) for committing this grave, major sin.

We have already referred you to fatwa 258232; so please reflect upon it.

Allah knows best.