How can one respond to the devil's whisperings that raise doubts about trusting in God and relying upon Him, and anticipating calamities and misfortunes, citing the killing of prophets and righteous people?

A Muslim is required to remember Allah often and to have a good opinion of Him. If Allah afflicts him with a trial, it is good for him, an expiation for his sins, and an elevation of his ranks. He must remember that these thoughts are from the trickery of Satan, so he must seek refuge in Allah from him, turn away from his whispers, and frequently engage in remembrance (dhikr) and seeking forgiveness (istighfar).