We played characters like this back when we first started gaming. We were about 12-13, and it didn't last too long. Oddly enough, the first thing we did when encountering new people outside of the beginning dungeon was try to talk to them. They attacked us so we went slaughtering things all over the place until we came to a town, then we calmed down and sold things and were generally good little adventurers.

Then we played Hollow World where we were rather specific about our targets, restricting them to one race (wood imps) and one human culture (Azcans). The things we did were...probably not stuff I should relate here but we were brutal.

It was fun. It was fun to do all sorts of disgusting and violent and evil **** (even if only one PC had 'Evil' on his character sheet - I was the poster child for "I'm not evil, I'm Chaotic Neutral"). But we were 12-13 and after a month or so of this we calmed down and worried more about local laws and culture and politics and diplomacy and story.

So if this is the first time the players go bananas like this, I would just sigh inwardly and allow them to run rampant all over the place and kill indiscriminately and rape and pillage and burn with no consequences, even alignment ones. Get it out of their system. After a certain amount of time, talk to them and say "Now it's time to stop playing crazy and start playing the game sensibly". If the players want nothing but to be killing machines, ask yourself if you want to DM that and if not, tell them you won't.