Stanza 15 Summary

Get out the microscope, because we’re going through this poem line-by-line.

If I've killed one man, I've killed two –

Here we are, back to the speaker's claim from line 6 that she killed her father. We know from that stanza that he died before she actually killed him. But here she is again, claiming that she's killed not one man, but two.
We can guess that the first man she claims to have killed is her father, and since the only other man in this poem is the model of her father, we're guessing that's the second man.
But we're also guessing she hasn't actually killed these men, except in her head.

The vampire who said he was you
And drank my blood for a year,
Seven years, if you want to know.

Now we get more indication that the second man the speaker has killed is, as suspected, the man that she modeled after her father and married. We're guessing this because that man, whom she's calling a vampire, "said he was you," so is very similar to her father.
She's already made this man out to be like Hitler, and now he's a vampire too. He's such a vampire that he actually drank her blood. She starts by saying that he drank her blood for a year, but then changes her mind and says he's been drinking it for seven.
Drinking blood could be a metaphor for the speaker's relationship with this man, which, as we found out earlier, was marriage. It sounds like this man has been draining her life away, like a vampire would drain his victim's blood.
We're not sure why she changed her mind on the time span. Maybe she was only married to the man for one year, but knew him for seven. Or maybe she thought he was only cruel to her for one year, but upon further thought, she realizes that he's really been cruel for seven, which could be the totality of their marriage.
(Interesting fact: In real life, Plath was married to Ted Hughes for about seven years.)
The speaker adds the "if you want to know" at the end of this line in what seems like a jab to her father, who could be either disinterested in or hurt by his daughter's distress.

Daddy, you can lie back now.

After the speaker has asserted that she's killed both her father and the man she married (who reminded her of him), she tells her father to lie back.
Normally you'd think of this as something comforting – like lying back and relaxing. But it's kind of weird that our speaker is telling her dead father, whom she seems to hate, or at least be angry at, to lie back and relax.
It's also a little strange that she calls him "Daddy," which is an affectionate name for someone she has aligned with so much evil.