Batman has never been the most stable of superheroes. His entire crime-fighting career is based around a deep childhood trauma and an obsessive need to punish the guilty. He's a man who will happily give up sleep, social interaction and even happiness to make sure that not a single criminal's jaw in Gotham goes unpunched.
A lot of the time, we see this behaviour as an example of what a shining paragon of justice the Dark Knight is. However, other times, Batman acts a lot more like you'd expect from a man who's hobby involves dressing up like a flying rodent and violently beating the mentally ill.
The Caped Crusader has always had a pretty dark streak (well except for that time he dressed up in a rainbow costume) and occasionally he'll indulge it, committing some pretty questionable acts in the pursuit of justice. And that's without getting in to the debate about whether or not he should be allowing young boys to fight crime with him.
From dealing with criminals in pretty horrific ways, to abusing and turning on his friends, sometimes Batman is almost as bad as his villains.