If God created everything, and he is omniscient meaning he knows everything that ever happened and ever will, and he is omnipotent, how does evil exist? Didn't he know when he created humans that he'd be sending them to hell? Is that not inhumane?

See, God may be omniscient and omnipotent, but humans still have some degree of free will, and in the very beginning of the Bible you see Adam and Eve choose to turn against God and sin. After that, they were cursed for turning away from God. After the original sin, humans were then destined to sin, which sinning itself is simply acting apart from God, or as though God doesn't exist. That is a choice of free will, to follow God or not.
Then, God send Jesus Christ to come down to earth, as a man, and die for the sins of all mankind, and because of this, no one HAS to go die, no one HAS to go to hell. Because of Jesus's death on the cross, anyone who believes in him and accepts him as their savior will have access to salvation, and to eternal life in heaven with God. That is more loving than anyone or anything on earth. And with such a gift of free salvation like that, it becomes free will to sin or to live without God, to decline the offer for free salvation, forgiveness, and eternal life. In the end that all comes down to believing in God strong enough and accepting him. Because he is a very loving God who WANTS to give us eternal life and have us in heaven with him forever.