An ex-hit-man comes out of retirement to track down the gangsters that killed his dog and took everything from him.

Just so you know, there is actually a category for "Examples"

As for your example of a John Wick logline, I think it's pretty solid. Short and to the point.

Here would be my try at it.

"When the dog his wife gave him is killed, a widowed hitman comes out of retirement to hunt down those responsible with extreme prejudice."