Created to be one, and the elements of his character reflecting that what you think of the character doesn't change that and as such it should be cast accordingly.
I think part of my reason to want an older Deathstroke is because I want diversification among the Anti-Batmen. Bane is the Dark Barbarian to Batman's Dark Knight, so he's an equal peer with radically different tactics and philosophy. Prometheus and Wrath are full on Evil Counterparts, with the exact same origin placed in a morally opposing context and eerily similar mannerisms and iconography. Hush (and Black Mask) are direct inversions of Bruce Wayne. And Owlman is the literal mirror universe counterpart often identical but twisted and distorted.
So to me, Deathstroke has always taken a cue from being Anti-Batman-but-more, and Evil!Captain America, which is just as much a part of who he was created to be as the anti-Batman concept was. I mean, look at Slade's costume and origins: he's an American combat veteran enhanced by super-science wearing blue scale armor with a preference for antiquated close combat weapons. That's all Evil!Captain America much more than evil Batman.
And if you'll permit me to give you a sales pitch for making Deathstroke significantly older than Batman, here it goes:
An older Deathstroke still benefitting from his enhancements into what should be his twilight years is literally
better than Batman is, making him a greater foe to Batman than even being a regular evil counterpart on steroids would be. This Batfleck is a 20 year veteran still in his prime for at least a little while more, but if you stack him up against a Deathstroke who's got 30-40 years of fighting under his belt and hits harder, faster, and recovers quicker thanks to steroids, than even the experience edge is lost and Batman is fully outclassed. Even his legend and psychological tactics are now challenged by someone who's simply racked up more victories.
Regular Deathstroke who's still roughly of Batman's generation is basically a more mercenary Bane. Older Deathstroke is a full on Titan God to Bruce's younger Olympian, larger, fiercer, and more primordial. Beating him takes on special significance then.
