Mercenaries should of course, above all, be mercenary. Since mercenaries of some description is a common career choice for PCs, this gives all the justification they need for ripping off paying clients, fleeing when the situation looks dangerous, and generally behaving in a slightly amoral way. It could even count as good roleplaying!

Better yet if they can make it sound like they're really doing what's best for someone else. Especially when it's actually just doing the best for themselves. Talent like that doesn't come cheap.

Transcript

Chewbacca: Luke, it has been a grand adventure, but here is where we must part ways.
Han: Yeah, seeya kid.
Luke: You can't leave now! You know what's about to happen, what we're up against. We could use a good pilot like... Chewbacca.
Chewbacca: As could many of our potential customers. However, in order to continue providing our unique services, we have decided our physical integrity is more important than our dignity. We wish you all the best.
Luke: I can't believe what I'm hearing.
C-3PO: Translation: They're great big stinking cowards.
Han: Well... I'm morally outraged that you would blow up an entire Moon full of innocent personnel just doing their jobs.
C-3PO: It's us or them!
Han: Or neither. Chewie, let's go.
Chewbacca: It has been truly a pleasure.
{Luke stares at them}
{Luke walks away}
Chewbacca: They have paid us, right?