As part of his seduction he was trying to stop the woman from killing the flea (”O stay, three lives in one flea spare,”) with his elaborate deceit. But when the woman tries to fend off his advances by killing the flea (”Cruel and sudden, hast thou since - Purpled thy nail in blood of innocence?)” he just uses her act to continue his metaphor in order to get her to “yield”.
in order to get her to “yield”.
It's what men have to do. :)