Even if humans have free will (and they don't, but that's a whole other story), it's very easy for us to be evil. That is an important point. We could've been designed so that it was a lot easier for us to make the right choices. We could've been smarter and less selfish as a species. Why would a creator choose to make us this way? The common answer is that it's some kind of test, but it seems to me quite twisted. We have to pass a test to be angels in Heaven and some of us will fail and go to Hell. Why didn't God just make a bunch of angels and leave out the testing and Hell?
What these discussions always seem to boil down to is that God has his reasons for doing things that may seem backwards and barbaric to us. It's simply beyond human comprehension why things must be the way they are. It might intuitively seem better for us to all be created as angels in Heaven, but God made things that way for a reason and we just have to trust him.