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.
It is proved in the sound Sunnah of the Prophet Muhammad that a person who is killed unjustly is considered a martyr, and so are those who are killed under building destruction or by fire.
Malik reported from the Hadeeth of Jabir Ibn 'Atik that the Prophet said: “There are seven categories of martyrs in addition to the one who is killed while fighting in the cause of Allah: (1) one who dies in plague is a martyr; (2) one who dies due to drowning is a martyr; (3) one who is killed of Dhat al-Janb disease (a disease that attacks ribs and causes inner ulcerations) is a martyr; (4) one who dies due to diseases of stomach is a martyr; (5) one who is killed by fire is a martyr; (6) one who is crushed under a wall is a martyr; and (7) a woman who dies while giving birth is a martyr.” [Abu Dawood and Ibn Majah]
An-Nawawi said while explaining Saheeh Muslim: “There is no difference of opinion that this Hadeeth reported by Malik is sound and authentic even though Al-Bukhari and Muslim did not report it.”
In another Hadeeth, the Prophet said: “One who is killed while defending himself; i.e., as a victim of injustice, is also a martyr.” [Ahmad and an-Nasaa-i]
Some Muslim scholars believe that one who is killed in a car accident is also a martyr since such a state is like being crushed under a wall, or like one who is killed in an accident and his car burst into flames.
However, there is no evidence that proves that the above-named categories of martyrs enter Paradise directly without being held for account. Such a state is in Allah's Hands. It will be enough to know that martyrs rank high in the sight of Allah Who promised to grant them Paradise.
For more benefit on categories of the martyrs, please refer to Fatawa 88945 and 5278.
Allah knows best.