Some have argued that evil does not have intrinsic existence, but is simply the absence of good; just as darkness is the absence of light. But whichever way you define it, evil still owes its existence to good—and the reverse. There could be no “good” if there were no “evil” that it could be contrasted with. When God created all things, and pronounced all that he had made “good” (Genesis 1); the first thing that he must have created, thought of, invented, imagined, or conceived of was the concept of good versus evil; otherwise he could not have pronounced what he had created to be “good”. The idea or concept of “good” cannot exist in anyone’s mind, including God’s, apart from the idea or concept of “evil”—and in contrast to it. That is the answer to the first question.
If God intervened to stop it every time somebody was going to do something wrong, that would deprive mankind of their agency. Man must be free to make wrong choices as well as right ones in this life, and thus to determine for himself what his ultimate end will be—even if that means that other people are going to get hurt by it. The good news is that the present state of affairs will not last very long. It will soon come to an end, and we will all enter an eternal state where we will each reap the reward of our labors in this life: “they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation” (John 5:29). In the meantime, repentance and redemption is granted through the Atonement of Jesus Christ for those who have sinned to repent of them and be saved, before the end comes when it will be too late. “Woe unto the world because of offences! for it must needs be that offences come; but woe to that man by whom the offence cometh!” (Matt. 18:7)
“Faith alone” don’t work; predestination is the doctrine of the devil; and Calvinism was invented by Satan to pervert the gospel of Jesus Christ. It is the biggest corruption and perversion of the gospel that Satan has invented since Christianity came into existence.