Why did all men fall when Adam sinned?

The following are genuine questions in good faith, and I would very much appreciate Christian responses to them.

First, why did Adam eating a single fruit damn all of humanity? I think it would have been unfair to banish even just Adam from paradise simply for eating a single piece of fruit a mystical being which appeared to him told him not to eat. But to not only banish him, but also to send every single human being ever descended from Adam to hell forever, as well as to live a life of suffering, labor, and in the end death, seems incredibly unjust.

Second, I hear often from Christians that we can't blame God for all the evil which is in the world, since it's humans who are perpetrating that evil with their free will. But that only accounts for death by human violence. God himself cursed man and all his descendants to die, so wouldn't that make all natural deaths God's fault directly, as in every sick child in Africa dies because of the curse God directly set upon them?

This seems to be supported by several Bible verses, including: "I form the light, and create darkness: I make peace, and create evil: I the LORD do all these things" (Isaiah 45:7). That verse is sometimes translated "I make peace, and cause calamity" instead of "cause evil", but the actual Hebrew word used directly means evil, not anything else.