This is not a slippery slope. This is an ice cliff.
As I noted yesterday, Joseph Carter wrote a fatuous essay in which he denies purpose in the universe and does an amusing dance around the implications that follow.
Aristotelian teleology is, as Joseph Carter points out, manifested by order in nature. More precisely, teleology is consistency: natural processes tend to consistent ends.
Jerry Coyne argues, astonishingly, that the vulnerability of handicapped children justifies killing them.
The Many Worlds Interpretation is sort of an extension of the Darwinian creation myth, applied to cosmology.