To gain entrance to Murder Pass, you need to talk to Mayor MacCready of Little Lamplight who may require the Lone Wanderer to rescue his friends that are trapped by the slavers at Paradise Falls before he will open the gate and allow your character through.
Tales of a Junktown Jerky Vendor on a toilet in the southeast corner of the area. Go straight from Little Lamplight entrance instead of crossing the bridge (to your left) where the Lone Wanderer first sees the super mutants. Look behind the guard station, it's on top of a toilet.
A close look at the architecture and debris within Murder Pass confirms that it used to be a part of Little Lamplight before the super mutants emerged from the vault. Furthermore, scaled down skeletons can be found littered around the ruins which one can interpret as the remains of former Little Lamplight residents.
If the option to open the back gate to Murder Pass is not available, you still can gain access to it by going through the Reactor Chamber and going in through the back door marked "Murder Pass".
In one of the huts in Murder Pass there's a broken terminal with a picture of Vault-boy taped to the screen.