When Bruce was around twenty, he traveled the world, going to Universities and studying forensic science and policing. He also learned from the masters of all sorts of fighting styles and man hunters. When he came back to Gotham City he found the Police Force (GCPD) was corrupt and there was even more crime. He decided to put his training to the test and go out into the world and do some good.
Bruce dressed himself up as a homeless person and found just what he was looking for. A pimp was beating up on a prostitute. Well, it didn't really turn out as planed. Bruce just barely avoided being arrested. After that whole fiasco, Bruce is sitting by the fire contemplating what went wrong, while quietly bleeding to death.
The main thing that bothered Bruce was that the prostitute generally kicked his ass. Bruce knows that he can kick anyone’s ass anytime (including the their's),but still they kicked his. Bruce thought that maybe if he could scare the criminals he would get an edge, the trouble was how. Bruce was thinking what to do, suddenly there was a loud crash behind him. Bruce whipped around to see a giant black thing crash though a window. That scared the snot out of Bruce for a second. Until he realized that it's only a bat the size of a small country. Time passes and Bruce realizes that if a bat like that would startle him, maybe a giant bat would make the criminals piss their pants. Thus he decided to become, a bat!
The whole bat idea worked really well. 'The Bat' would suddenly appear in front of the criminals, scare the snot out of them, kick their butts and disappear as suddenly as it came. The thing that worked the best was that, 'the bat' was a legend. The criminals thought that it was supernatural, if they even thought it existed at all. Eventually they came to call it 'The Batman', and then just Batman. The sword was double-edged though, while Batman had the Criminals scared, he also had the police chasseing after him. Each side though that Batman was out to get them.