What makes Deathstroke a compelling character and a fan-favorite? It would be all the nuances this movie missed, even though you can tell they tried to capture those nuances while making Slade a "hero". But making him a hero is where the movie failed.
Deathstroke is neither hero nor villian. He is an ethically failed character but with a strict code of morals and values. He strives to be the best there is at what he does, but also fiercely loves and is dedicated to his family. This movie sets him up to be a confused hero, which he is not. While he does often choose to avoid full-on evil/sadistic employers, he isn't "virtuous". It also misses that while Deathstroke is a master warrior and tactician, he is also a master strategist. This movie basically has him always taking bullets, getting beat up, and one-step behind the enemy at every stage.
How they chose to craft this story is also baffling. Why take such a complex character with years of great stories to pull from; and slap together this nonsense? I guess they were trying to take a "fresh" approach, but they easily could've just crafted a story with characters true to their lore while still making it feel relatable enough to sell to the casual watcher.