In 1950?s Los Angeles a vengeful rookie DA learns to think like the notorious criminals he prosecutes in order to bring justice to the Southland.

Think like a criminal is way to vague and doesn't say enough. Does he become violent or mean or start taking drugs. It doesn't say enough for me to form an opinion on the story.

I worked in intelligence and ran several investigation teams. If anyone is interested in how an investigation is run email me. craig.griffiths@askfindbuy.com it might help anyone wanting to write real cop stuff.

Good point by DPG also add in a clear goal, what is it he actually needs to achieve in order to catch the bad guy.

I think "...vengeful..." gets used a lot as a character description because it implies the character is generally motivated but the problem is it is a generic description on its own.

In what way is he vengeful? What made him vengeful that will be different to the many other vengeful main characters in many other stories?

For example if you were to describe him as a vengeful widower the reader can draw the conclusion that he is vengeful for the death of his wife. Or vengeful grieving father could mean he is vengeful for the death of his child then you can tie this in with his goal to stop other wives or children being killed.

Other wise he is just out on his own pissed and on a mission for no apparent reason and this could render him crazy or un-sympathetic.

Hope this helps.

"Thinking like a criminal" is standard operating procedure, what every law enforcement person has to learn to do to be effective. So this is not a salient characteristic that would set the DA apart from the pack, that would distinguish the concept from all the other crime procedural series.