Then why did god create humans such that they could sin (and would sin, as an omniscient god knew full well they would) and then punish them for it?

It seems sadistic and perverse, and yes, it leads one to question whether god is really benevolent after all (which, if he/she/it is not would likewise shatter most major religions).

Of course religion has answers for all of these questions... mainly having to do with humans having free will or evil being a "privation of good" (ie. there's no such thing as evil, but only a lack of good), etc.. but that's where the arguments start.