Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- The obvious reason given is that public interest requires that the killer be hindered from inheritance; since, if he was to inherit, killing would become widespread and thus lead to universal chaos.
3- However, not every type of killing or “causing” of death is a hindrance to inheritance.
In his answer to your question, Mufti Muhammad Ibn Adam Al-Kawthari, Director and researcher at the Institute of Islamic Jurisprudence (Darul Iftaa, www.daruliftaa.com), Member of the Al-Qalam Shari`ah Scholars Panel, and advisor on Islamic Banking, states:
Abu Hurairah (may Allah be pleased with him) relates that the Messenger of Allah (peace be upon him) said: “The killer shall not inherit.” (Ibn Majah)
Based on this Hadith and other evidences, it is a general principle among the jurists that a killer does not inherit from his or her victim.
The obvious reason given is that public interest requires that the killer be hindered from inheritance; since, if he was to inherit, killing would become widespread and thus lead to universal chaos.
However, not every type of killing or “causing” of death is a hindrance to inheritance; rather, there is some detail in this regard.
It is stated in the renowned Hanafi Fiqh reference work, Al-Fatawa al-Hindiyyah, 6:456:
“One who kills unjustly does not inherit anything from [the estate of] the one killed according to us, whether he killed him deliberately or accidently…”
It is further explained, as a principle, that a killing which in-of-itself necessitates the law of retaliation (qisas) or expiation (kaffarah) is considered “direct” killing, hence a hindrance from inheritance.
However, a killing which in and of itself does not necessitate the law of retaliation or expiation is considered “indirect” killing, hence it will not be a hindrance from inheritance.
The law of retaliation or expiation are necessitated in the following three ways. As such, a killer does not inherit from his victim if he:
a- intentionally kills his victim with a tool/weapon that would normally kill, such as a gun, sharp knife, sword, or a heavy sharp rock (qatl `amd).
b- intentionally kills his victim with a tool that normally is not used to kill someone but it killed his victim nevertheless, such as a stick, small stone or strangling (qatl shibh `amd) (e.g. intentionally running someone over with a car and killing them would also fall under this category).
b- mistakenly kills his victim, such as, in a hunting session, trying to shoot an animal, he mistakenly killed his victim (qatl khata’) (e.g. mistakenly running someone over with a car and killing them would fall under this category).
If the above three types of killing took place at the hands of a mature and sane person, then he will be deprived of inheritance from the estate of his victim.
However, if death was “caused” in other than the above three ways, i.e. it was indirect, then the killer will inherit from his victim’s estate. For example, a son dug a hole in someone’s garden and his father fell into it and died. The reason being is that this “causation of death” is “indirect” killing which does not necessitate qisas or kaffarah, and as such, it will not hinder one from inheritance. (Al-Fatawa al-Hindiyyah, 6:454)
Almighty Allah knows best.