See, Benedict Cumberbatch will inevitably bring an essence of familiarity (It always feels like Cumberbatch playing a character, not just the character). Keanu Reeves can disappear into a role. In The Matrix, he's not Keanu, he's Neo. He actually just is. In John Wick, he's fucking John Wick and in Bill and Ted, he is Ted.

That's the part where I think Keanu Reeves as Dr. Strange would be an awesome Dr. Strange, where Benedict Cumberbatch would just be "Cumberbatch being Dr. Strange".

Now, let's get to the "dirty" part. The following are "dirty" characters (off the top of my head - not necessarily villains):

Phillip Seymour Hoffman in Mission Impossible III.

Matt Damon in The Departed

Heath Ledger in The Dark Knight, prime example here.

Rorschach in Watchmen

Kevin Spacey in The Usual Suspects.

Christian Bale in The Prestige.

Anthony Hopkins in Silence of the Lambs (hell, even in a comedy movie like RED 2, he manages to go full crazy - it's crazy how well this guy does it)

Hans Landa, obviously.

Examples of non-dirty villains:

Most bond villains.

Cumberbatch's Khan.

Bane.

McKellan as Magneto.

I think it's the difference between how much dignity the actor brings into the character. Anthony Hopkins sheds dignity like a snake before entering his creepy as fuck roles.

If you watch The Talented Mr. Ripley, Matt Damon is ridiculously creepy in the movie - he sheds any connection to his real life as an actor and becomes fully on psycho - it's unsettling from the first 5 minutes onward until the end of the movie. That is a very good example of being a "dirty character" in way I I think I mean it.

Compare McKellan's Magneto with Fassbender's. Grant, they are written differently but I can't imagine McKellan doing what Fassbender does. Fassbender embodies a pure rage in his character. McKellan's Magneto is way too dignified for that shit.

In summary, Benedict Cumberbatch is doomed to bring some dignity into every character of his and so far always brings "Benedict Cumberbatch" into the damned roles too. Some actors can avoid that, Keanu is one of them.