At best you're going to be stuck with someone that's pissed you killed her cat. Even if she sticks to the deal she's going to make your life miserable. And if she is the cat ...

If you can't charm her: tranquilizer dart gun. Then a romantic getaway at your hidden base that's also an active volcano. Apologize for your crude methods, but justify them. Point out how horrible those other men were, in fact how horrible humanity is as a whole. "We're not so different, you and I", etc.

Wine and dine her while you explain your plan for world domination. You want the hot shapeshifter on your side -- don't screw it up.

I mean, if she is the cat, and you kill the cat and take the key in order to marry the woman, then you will be marrying the corpse of the woman you just killed. Seems like it might be a bit of a plot point that you don't kill the woman (or do, but that's a very different film.)