He is 'seeking revenge for' his father's murder. [=he wants to harm the person who killed his father]
He swore to 'take (his) revenge on' his enemies. [=to harm his enemies]
The bombing was 'in revenge for' the assassination of their leader.
(..) He had to. If he did, he should have told the people. Everyone lost relatives. Fathers and uncles, sons and daughters. They would have demanded revenge. That is why he didn't tell them. - What do you mean? - It was a waste of time. They were dead! Ragnar wanted to sail to Paris. (..)2016 TvShow: Vikings Title: The Last Ship Season: 4 Episode 10