Why is a martyr called a "shaheed" (martyr) in Islam?

Scholars have differed regarding the reason why a martyr (shaheed) is called a shaheed. Some say: it is because he is alive, or because Allah and His angels bear witness for him to Paradise, or because he witnesses, upon the departure of his soul, the honor prepared for him, or because safety from the Fire is witnessed for him, or because there is a witness over him that he is a martyr, or because none but the angels of mercy attend him at his death, or because he will bear witness on the Day of Resurrection to the apostles' conveyance of the message, or because the angels bear witness to his good end, or because the prophets bear witness to his good adherence (to their teachings), or because Allah bears witness to his good intention and sincerity, or because he witnesses the angels at the time of his agony, or because he witnesses the dominion (malakut) of this world and the Hereafter, or because safety from the Fire is witnessed for him, or because there is a sign upon him bearing witness to his salvation.