I think that what the beaver was trying to say was that Aslan is one to be feared because he has the power to do anything, just like who he is representing, God. We are to fear God because if we do anything the Bible says not to then we should be afraid of what God’s wrath will do to us in this life or the next. However, God is most definitely good; He only punishes where punishment is due, not out of spite like Satan (the Witch). Aslan is proven good later on because his desires are to help the Pevensies, and to put an end to the Witch and her bondage over his people.