(Spoilers Published) In ASOS, what is Melisandre playing at here with the Onion knight?

>"Is the brave Ser Onions so frightened of a passing shadow? Take heart, then. Shadows only live when given birth by light, and the king's fires burn so low I dare not draw off any more to make another son. It might well kill him." Melisandre moved closer. "With another man, though . . . a man whose flames still burn hot and high . . . if you truly wish to serve your king's cause, come to my chamber one night. I could give you pleasure such as you have never known, and with your life-fire I could make . . ."

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- ASOS Davos III

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I've often wondered what was Melisandre trying to do here in her attempt to seduce Davos? He has no king's blood that we know of, so if the rules work how we believe, they shouldn't be able to make another shadow assasin right (and if they can, why doesn't she just go to anyone else after he turns her down)? Was it just an attempt to get on the good side of the one man she knows has the full confidence of Stannis? Was she trying to sow division between Davos and Stannis? Is she luring him to somewhere private so she can kill another person trying to kill her? Is she just testing his loyalty to Stannis?

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Trying to seduce the person Melisandre knows wants to murder her has always seemed like such a confusing play to me. So what were Mel's motives here?