In this story, Slade Wilson is a mercenary-for-hire and a key figure in the current murder spree in Gotham. Slade Wilson has been doing throwaway jobs for years and stayed out of Gotham while Batman lost his moral compass. Recently, Slade approached someone who wanted to clear Gotham of its greatest players: politicians, business owners, cops, and a certain masked vigilante. They called it the "Gotham Contract", a list of targets that no one dared to take out in one go. Slade was intrigued... and after seeing the Bat's return to glory, what better time to strike than now? Deathstroke's goal is to complete his job. No matter his obstacles, Slade won't stop until his contract is complete. A high paying job like this is one you can't just turn away. And a shot at the Batman? A match made in heaven, really.

Slade only has one line in Justice League (I'll leave a link at the end of this description). That being said, it's not a hard voice to pull off. I'll go more into it in the next paragraph.

Slade has a gravel-like voice. It's rough, but it also stings; he speaks harshly, always getting to the point. His actions shout efficiency, and so do his words. His voice should be as menacing as his methods.

Deathstroke reference:

https://www.youtube.com/watch?v=Twanwp3F7Z4 (Line at 1:08)