Any rogue needs a well-considered reason for entering the rogueing profession. It's not an easy life, all appearances in fantasy and science fiction literature to the contrary.

For a good roguish background, simply adapt whatever the PCs did to some random group of NPCs in a previous campaign.

Transcript

Leia: I cannot believe it. That freaking chicken-liver is too scared to use the Force.
Leia: And the last Jedi Master, back from exile, and these bozos let him die.
Leia: Obi-Wan Kenobi. He was our only hope.
Chewbacca: No. There is another.
Leia: A Jedi? Who?
Chewbacca: His name is Yoda. He survived the Silence of the Clones.
Leia: Where is he?
Chewbacca: He sabotaged my standing amongst my people on Kashyyyk, my home planet, forcing me into a life of...
Chewbacca: Well, this.
{shot of Han}
Chewbacca: Then he took off in his little spacecraft and stranded me amongst my now hostile peers. Thence, I know not whither.
Leia: Man, what a freaking muppet.