I've been thinking about this for a while now actually. Start the film with the origins of him in the middle of a war. Meet his wife, lose his friends in war, get offered a mercenary contract. Then skip forward a few years and he's living at home with his wife and 4-year-old son.

His wife finds out that he's still an assassin for hire. Calls the feds and pulls a gun on Slade. He tries to reason with her and exclaim that his work is more honest then taking loan after loan from the Mob. She doesn't take and continues to point the gun at Slade. Slade threatens her and tells her he'll never stop looking for his son. As Slade turns around Adeline (his wife) shoots him in the head. Slade falls to the ground with his hands covering his eyes.

Skip forward a couple months and we cut to Slade interrogating a fed to get further information on his wife. After the guy says the city she's in he kills him in cold blood. (This Slade is not a hero. He is in this for personal reasons and some should be able to view him as a villain. Unlike Suicide Squad)

Next scene is Slade sitting in a car with a full beard. The car is littered with fast food and beer cans. He's been watching the witness protection house where his wife and son have been for a while. It's nighttime and Slade is starting to doze off as he hears shots from inside the house.

Slade storms the house with an Assault Rifle. He finds his wife bleeding out. She's pleading for help as Slade walks down the hall to find his son. He opens the door and finds the body.

He tortures the men until they tell him it was a mob hit by the Maroni Family. He then chases down all possible leads to the mob boss Luigi Maroni. The film takes on a similar tone to John Wick and becomes a full on revenge film.

This is just the beginning of it. I had a couple other things planned like Nightwing showing up half way through.