A martyr ("witness") is someone who suffers persecution and death for advocating, renouncing, refusing to renounce, or refusing to advocate a belief or cause as demanded by an external party. In the martyrdom narrative of the remembering community, this refusal to comply with the presented demands results in the punishment or execution of an actor by an alleged oppressor. Accordingly, the status of the 'martyr' can be considered a posthumous title as a reward for those who are considered worthy of the concept of martyrdom by the living, regardless of any attempts by the deceased to control how they will be remembered in advance.
Yes, there are many authentic Hadeeths from the Prophet Muhammad (SAWA) and Ahlul Bayt (AS) stating that all of the infallible will be either killed or poisoned (ما منا إلا مقتول أو مسموم).
This Hadeeth was narrated by Imam Hasan (AS) from his grand father the Prophet (SAWA) after the martyrdom of Ameerul Mo'mineen ( AS). (Kifayatul Athar, by al-Khazzaz , page 160-162).
The Prophet (SAWA) himself was also poisoned. There are plenty of authentic narrations in Sunni and Shia books stating that the Prophet did not have natural death but was poisoned. ( Al- Mustadrak by Al-Haakim al-Nisaboori, V.3, P.60) and (al-Seerah al-Nabawiyyah by Ibn Katheer , V. 4, P. 449) and (al-Tabaqat al- Kubra by Ibn Sa'ad , V. 2, P. 249). These are few of Sunni books.
Thank you for your question. In the same way that we mourn departed loved ones out of the love and mercy for them that exists in our hearts, even if they may have been martyred, so too the lovers of Ahl al-Bayt (as) mourn the terrible tragedy that befell them (as).
That certainly does not take away from their reward, rather through God’s Mercy, their tribulation and the softness of our hearts towards them, and the mourning that is a natural consequence, has been validated and made into a worship by God. Crying for the tragedy of Imam Husayn (as) is praiseworthy and has been associated with great rewards in our scriptural literature. It is from here that the tradition of mourning finds its roots.
May you always be successful
The martyrdom of Imam Husain (AS) gave the Muslim Ummah the biggest shock which saved the Ummah from collapse when the rulers (Yazeed and his gangsters) were totally away from the teachings of Islam. Muslim masses could not accept the crime of the killing of the grand son of the Prophet along with his family members including children and imprisonment of the family of the Prophet (SAWA). This greatest shock brought back the sense to the Ummah and was the reason behind hundreds of revolts against tyrant rulers during centuries of the history of Muslims. Imam Husain's martyrdom declared the tyrant rulers as deviants and made the Muslim masses careful when dealing with them.
The martyrdom of Imam Husain (AS) became the guide to his noble value which calls Muslims to be real Muslims and join good and avoid evil. Thousands of scholars, writers, poets were inspired by Imam Husain's martyrdom.
Millions of Muslims all over the world and all over the generations, observe the martyrdom of Imam Husain every year and get spiritually charged to be better Muslims.
Imam Husain's martyrdom has inspired many non Muslims and made them realize the great teachings of justice for which Imam Husain (AS) has sacrificed.
The list of the benefits from Imam Husain' martyrdom will be very long and books can be written on it, but to cut it very short, we need to remember the Prophetic Hadeeth which stated: Husian is from me, and I am from Husain. How the grand father is from his grand son? In fact, he religion of Islam which was sent through the Prophe Muhammad (SAWA), was revived and made everlasting because of Imam Husain (AS).
Reviving Islam is the great benefit not only for all Muslims but for all mankind.
The narrations regarding history matters are usually different, that is why you find in matters of history, deferent opinions. The most famous opinion among the historians that the number of the martyrs of Karbala is 72 martyrs.
We have other narrations in some books suggesting different numbers.
We don’t have such claim regarding cancer in our authentic books of Hadeeth.
We have in Sunni and Shia books that every lover of Ahlul Bayt ( AS) dies as Shaheed ( martyr). من مات على حب آل محمد مات شهيدا
Tafseer Al-Kashshaf, Tafseer Al-Razi and other Sunni books.
I have discussed this topic in detail in my brief series on Life after Death:
Based on certain verses, like Surah al-Tawhbah, verse 111, and Surah al-Saff, verse 10, it is understood that Almighty God forgives the sins of a martyr who dies in the path of God. The martyr will be blessed with the everlasting abode of heaven because of their sacrifice.
Of course, we know that not every person will have the great opportunity of dying in the path of Allah, and it is indeed something granted by the Almighty to only those who deserve it and have spiritually worked for it.
We also have traditions that refer to certain individuals who will enter Heaven without being judged, and there are plenty of such cases.
If the martyr has certain obligations towards others, or a debt, or something like that, then that would also not be in conflict with them entering heaven.
And Allah knows best.