I agree with a lot of what has been said here in terms of tips. There's no way their murderous actions would go unnoticed. I'd hit them hard with multiple set back dice on most social interactions with any moderately sane people. I'd start having their contacts dry up and hit them pretty heavily with imperial intervention. If they're constantly running into heavy armed imperial forces and they're forced to run away from most fights they might start to see they need to probably change things a bit. Again, the most important thing is that your players are having fun so whatever works for you.
Personally, I find players that just want to be assholes and murder everyone are below the maturity level that I want to game with. Being purely evil is just so "easy" that I can't imagine doing it. I mean, just shooting every one in the face instead of interacting with NPCs and engaging in dialogue really limits the scope of the game. The pre-written adventures are filled with clever openings during interactions that allow for side quests or hidden bonuses. Edge of the Empire is awesome because it presents so many opportunities to "ride the line" with morality. Your characters aren't perfect angels. They come with a back story and substantial obligation from their "past life". They're dealing in drugs, killing gangsters, thugs and crime lords left and right. They can be cutthroat and tough but still have a modest conscience (whether from their own moral choices or out of fear of authority) .
Didn't mean to rant, and to each his own. That's just my two cents.