Good question Aphie. Before we get going on another big one, let me add to your questions: What is a "force"? What is "metaphysical"? What is a "metaphysical force"? What is meant by "exist"? Perhaps as a refresher also: what is meant by "force" in physics?
I like the Buddhist views of "construction / destruction" best. If you observe the universe, it seems like a constant cycle of constructive design followed by the occasional super-nova that resets everything. Even the universe itself may be just a bubble from a much larger quantum sea.
Of course they exist, just look around - Good - Stargate Command, Starfleet, Dumbledore, the Charmed Ones, Cubs, Paula Abdul Evil - Go'uld, Borg, Voldemort, the Source, Sox, Simon Cowel OK, maybe the last one in each category should be flipped.
things do need to be defined more to answer this correctly. i don't believe in like an evil entity that occupies people, as in like posessed by the devil, etc. however, people can obviously be cruel. but even if someone is cruel, there are probably explanations why. while on the subject, i don't agree with bush's "good v. evil" stuff.