Where Did the Race of Men Come From in The Silmarillion and Why Were They So Easily Corrupted?

 The Race of Men is kind of an oddball race in The Silmarillion (A book that narrates the whole history of Tolkien's world). One of the first times we see men pop up in Beleriand is when a group led by their chief, Balan crossed the Blue Mountains in the east. Apparently, men were created far to the east, unlike the Elves and Dwarves who awoke in the west. This group was discovered by the elf Felagund, the King of Nargothrond, and a member of the Noldor (In other words he came from the elves that lived in Aman instead of having lived forever in Beleriand). Balan was renamed Beor at this point and his people came and lived with the Noldor. The Sindar (The elves who had lived forever in Beleriand) wasn't all that fond of these visitors and kicked them out of their kingdoms. After a while, however, we see that the Dark Lord Morgoth (Basically Tolkien's equivalent of Satan) thought he might be able to take advantage of this new race. When Felagund was warning Beor and his people about Morgoth, a figure arose that looked like a man named Amlach. This figure essentially said that the Elves were liars and wanted to destroy men. When Amlach actually showed up he denied having said anything of the kind. Beor, apparently deciding that these parts weren't particularly friendly took a thousand of his people and crossed back over the mountains. A short while later the majority of the remainder left and followed him. Those that stayed behind would meet with various misfortunes but were the ancestors of the heroes Beren, Tuor, and Aragorn. The people that crossed over the mountains apparently returned to the area that would become known as Middle Earth! Thus, these people must have been the ancestors of the many races of "wild" men we meet in the Lord of the Rings, including the savages that helped the riders of Rohan with intelligence, Beorn and his followers, and the many settlements scattered in the wide expanse between Mirkwood and Gondor. As for the second question, here's where Tolkien's story begins to mimic real life. Morgoth left his fortress for a short while to go the east where Beor's people had originally lived. The theory goes that men had already fallen to Morgoth's lies, which is why a "shadow was upon them" in The Silmarillion. They had already fallen once and thus were much more likely to fall again. Evil was already in them since the day they first appeared because of Morgoth's lies. The exact same thing happened in the Bible. Adam and Eve were corrupted by Satan's lies, and all of their descendants had that shadow in them. God's perfect creation was stained, and we can see that reflected in Tolkien's own work.