You see it all the time. The character of Lovejoy in season one is a tough guy, even has the odd run in with the police, yet by the series finale you’d invite him home to meet your favourite aunt. Similarly Dexter, who was an asexual monster who pretended to have feelings, became quite touchy feely by the final season, even had genuine emotions. It’s not just main characters. Jayne Cobb, the semi feral henchman in Firefly was a mean hombre in the pilot, would cut anyone’s throat for the right price. Yet by the final episode he was a fun character who’d you let date your sister.
It’s understandable. For every hour you sit putting a character into the written word, you’ve usually spent another four hours thinking through their dialogue and behaviour. Often a character will get more thought than a real life partner, so it’s not surprising that writers start to really like their characters.
That is the problem! Once you get a soft spot for your character you can’t resist making them more likeable, add emphasis to their good points, and have them do a good deed. For the sake of your own sanity avoid the good deed plot twist! Once you go down this route your dark edgy character becomes about as menacing and credible as Hannah Montana.
Don’t get too fond of characters, let them have flaws. Even try to kill off one or two that you actually like, it’s known as ‘doing a Whedon.’ If you find a character is getting too nice, or that your test readers are growing attached to them, give them back their edge; give their worst character flaw an outing.