Be after (one's) blood British phrase
To look for a chance or want to harm someone because they did something bad or wrong to you, your family or friends.
She has been after this man's blood as she believes that he was the one who killed her spouse with impunity.
She is after his blood as he betrayed her.
Her ex-boyfriend was after her new husband's blood for intense jealousy.
To retaliate against someone.
To treat someone badly in the same way they have treated you
To act in retaliation against someone; to punish someone
Get revenge.
If you revenge yourself up(on) someone/something, you cause harm to someone/something in return for something bad that they have done to you.
The verb "be" must be conjugated according to its tense.
The origin of this phrase is not clear.