Let's also remember that the island showed Boone his sister's death, and Locke knew that (not to mention Jack's dad). So not all the island visions come true. Also, Locke's dream with the plane, his mother, the wheelchair, and bloody Boone definitely didn't make it seem like the choice was between the wheelchair and sacrificing Boone.
It has occured to me, though, that the island might be evil and is basically turning Locke by giving him gifts that it will threaten to take away.