Is suicide permissible in Islam if it is for the sake of Allah, and what is the difference between it and casting oneself into destruction?

A Muslim is permitted to manifest his religion and speak the truth, even if it leads to his death. This is not considered suicide, for suicide is when a person directly takes his own life. The case of a Mujahid (one who strives in the way of Allah) who patiently fights the disbelievers to uphold the religion, even if it leads to his death, is not suicide.

As for the Dajjal's (Antichrist's) trial, a Muslim is commanded to choose the Dajjal's "fire" because it is salvation. A Muslim does not choose destruction but rather salvation. If it is destined to involve the killing of the soul, then this falls under the category of prioritizing the destruction of the soul over the loss of religion and disbelief, for fitna (polytheism/tribulation) is more severe and greater than killing.