This kind of hitman in movies is the prototypical anti-hero. He is dark because he must be sneaky; he is mysterious because he can't have personal attachments; he is excellent at what he does because he is still alive to tell the tale. (Which he doesn't. Except to that one beautiful girl who will never see him again.) We like watching anti-heroes, such as the protagonists in Mad Men and Breaking Bad, because they allow us to feel better about our own bad behavior.
But there's a key ingredient when it comes to the hitman: He isn't meant to show emotion.
Think about Don Draper (Jon Hamm) for a second, who only ever killed someone in a dream. Yes, he's admirable for the suave coolness he oozes wherever he goes. But in addition to that, we watch him go through the full gamut of human emotions. We see him angry (a lot), we see him sad, we see him nervous, we see him having a nervous breakdown. The ideal hitman shows no emotion. Angelina Jolie and Brad Pitt in Mr. and Mrs. Smith. Colin Farrell in London Boulevard. Christian Bale in Equilibrium, who had to be given a puppy for us to like him. Arnold Schwarzenegger as Terminator is the best example - he's a robot!
What I'm getting at is that while it's very cool to have a hitman in your movie, and I certainly am not immune to the charms of watching someone decimate an entire room of people with perfect fight choreography, the problem with having a straight up hitman as your main character is that they aren't very interesting as people. I have no emotional connection to them and no investment in their fate. Even Keanu Reeves in John Wick was entertaining, but not particularly compelling.
When you can't relate to a character emotionally, you're missing a pivotal element to the narrative: Why do I care about this person? Yeah, he's fun to watch fight. He's fun to watch saving people. But I don't get much else out of it, let alone the stamina to sit through 2+ hours of him fighting and saving people and not caring.
On the flip side of this (very rudimentary) theory, you have Kingsman: The Secret Service. Remember Christian Bale's puppy? Well, in Kingsman main British guy Eggsy (Taron Egerton) gets his very own puppy to train and develop with at his super secret spy agency. Now, Eggsy clearly does not have the makings of a super spy. He didn't go to Oxford or Cambridge. He doesn't have an impressive family lineage. Even his father, the reason Eggsy was accepted into the training program in the first place, died in the line of duty early on in his career. And to top it all off, Eggsy can't even shoot his stupid dog at the end of training – which, as he's later informed, only had a blank in it!
But the fact that Eggsy couldn't shoot the dog and didn't have the credentials to be a Kingsman is what makes him an interesting character. I could have listened to him goad people to fight in that British accent for hours. It was funny to see him revert back to his sartorial roots for a mission where he had to be a pickup artist. And when he ultimately saves the world, his motives stem more from avenging the death of his mentor than they do from a desire to actually save the world. The reason Eggsy is compelling is exactly because he didn't make for a good hitman. He doesn't blend in, he's terrible at playing nice with others, and above all – he displays emotions. Lots of them.
Video games, though. Now there's a place where a well-appointed, emotionless hitman can have it all.