But as far as the idea of paying for them after you dismiss them, yes. [...] OR, if you are a shady, evil type, you could kill them and take all their stuff. That's entirely up to you, the player. My evil drow charlatan sorcerer would absolutely order Lae'zel to kill some merc who signed on thinking he was going to get a fair share only to discover he was in a party full of evil people who would use him and murder him when they were done with him. Shoot. Whether you pay up front or if you want to dismiss them, I think that option would TOTALLY make sense..
If you could just kill them to avoid the cost of hiring them, I'd want there to be some consequence. Maybe some sort of reputation system for hiring mercs. E.g., the next merc you attempt to hire *does* require (some or all) pay up front, possibly at a higher rate, because the previous merc hired by your party mysteriously disappeared. And if you kill this one too, then you're blacklisted and can no longer hire mercenaries.