All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Islam considered killing a Muslim unjustly prohibited and labelled this act as one of the major sins, since Allah, The Exalted Said (What means): "Whoso slayeth a believer of set purpose, his reward is hell for ever. Allah is wroth against him and He hath cursed him and prepared for him an awful doom." [An-Nisa/93] and He enjoined the reward of a martyr to the person who was killed unjustly and perfidiously and the same applies on the one who was killed while carrying out his duty.
Further, the reward of martyrdom is not only for the one who was killed for the cause of Allah but also for the one who was killed unjustly, as clarified by the Prophet (PBUH) to the companions when he asked them?: "Whom do you consider to be a martyr among you? They (the Companions) said: Messenger, of Allah, one who is slain in the way of Allah is a martyr. He said: Then (if this is the definition of a martyr) the martyrs of my Umma will be small in number. They asked: Messenger of Allah, who are they? He said: One who is slain in the way of Allah is a martyr; one who dies in the way of Allah, is a martyr; one who dies of plague is a martyr; one who dies of cholera is a martyr." [Related by Muslim]. That’s why Al-Imam Ar-Ramli stated on the above narration as follows: "the one who was killed unjustly is to be considered a martyr and has a similar rank of the martyr in combat or for the cause of Allah; yet the rulings of the latter in this life can't be applied on the former…" [Nihayat Al-Muhtaj vol. 2, pp.496].
Although the reward of martyrdom for whom was killed unjustly is guaranteed; yet its rulings as regards the one who was killed for the cause of Allah during worldly life won't be applied on the former since martyrdom is a reward allocated for him by Allah in the hereafter only. In this regard, Al-Imam An-Nawawi (May Allah have mercy on his soul) stated: "There are three kinds of martyrs as follows: The first kind is the one who was killed in combat or as a result of it; thus, he will have the reward in the hereafter as a martyr in combat for the cause of Allah and the rulings of worldly life regarding the dead and other kinds of martyrs won't be applied such as: washing the dead and funeral prayer. The following kinds are considered martyrs; yet, the aforementioned rulings regarding the dead are to be applied such as: The second kind: is the one who will have the martyr reward The one (a Muslim) who dies of an abdominal disease, the one who dies of plague, the one who dies under the debris (of construction, etc.) the one who is killed protecting his property and many others who were mentioned in sound Prophetic narrations as martyrs; however, the superiority of reward is to the first kind. [Sharih Muslim vol.2, pp.164]. And Allah Knows Best.