God kills Er

Genesis 38:7 says, “And Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him.”  What is the point of slaying Er?  He is no part of any other story here so it isn’t like we are learning something from it.  If God is all-knowing, why would he create a child who was wicked?  Or if he knew he was going to become wicked and that he would have to slay him why bother making him at all.  If I knew everything and knew one of my creations was going to be bad I would make it good or at least not make it at all?  I guess it isn't the first time god has killed man.  He wiped out almost all of the earth, including innocent animals, a few generations ago.