The absurdity of life is the absurdity of existence: that there is anything at all is completely absurd and it's doubtful that this will ever be explained satisfactorily.
But what bugs me most is that a universe without an entity to perceive and observe it is just such a waste of ... energy? It's almost like the existence of the universe implies the existence of an observer. That the formation of intelligent life is an expected result of the universe existing.
It can only be a waste if it has some higher purpose.
Seems like a logical stretch, but very poetic.
Entropy leads to life. Life and entropy leads to evolution. Evolution, given enough space and time to work within, eventually leads to intelligent life.
Intelligent life leads to...?
Spoilers!
So that's probably not what you consider "optimal ethics", but given the starting assumption (that there is no God), it seems very difficult to explain why de Sade was wrong.
If we can't prove unambiguously whether or not God exists (and we can't), much less what the actual Will of that God is (even the religions which more or less agree on the same God can't agree on that), then ethics in the presence of God are in practice no different than ethics in the absence of God.
I think it's difficult to prove the Marquis wrong because it's impossible to prove him right.
Edit: let's just pretend I wrote this from an alternate mirror universe where what I said here made sense in context. I'm not deleting this comment because that's the coward's way out....
And many people accept de Sade's premise. They are then left with his conclusion. (Not necessarily sadism - that's just what de Sade felt like doing. The part that people are left with is "Whatever is, is right" - that is, there is no basis for a "should" or an "ought"; there is no basis for morals.)
Morality doesn't require the universe have a purpose, but morality has to have a purpose.