For people to debate you, you need to make a claim. Asking a question doesn't work.
To answer your question. No it wasn't justified.
Understandable? Sure. But not justified.
There is no good justification for revenge killing. It's literally just a version of "Because I felt like it."
The deputy he ran over, wasn't even the police officer that shot his son. Those are different and unrelated agencies. So he didn't even get revenge properly. He just murdered a random cop. Because he felt like it.