Sanders thinks -- totally wrongly, and the audience knows he's mistaken -- that del Rio set him up to be robbed of nearly a million dollars by Leon Ames. So, when he comes to (after being knocked out in a Japanese bombing raid in Shanghai), del Rio is gone (to get a vehicle to help him get away from the building that had been bombed) and he has no idea that her blood transfusion saved his life. So for the next few scenes getting toward the end of the film, and until June Lang explains what del Rio did for him, he is resentful toward del Rio. This, of course, has nothing to do with her actually being a femme fatale. She's the polar opposite. After he listens to Lang, Sanders can't wait to go to del Rio because he's back to where he was before: in love with her (as she is with him).