This is how it appears to me, from the player's perspective: We have a map to an old lair. It used to have a witch, but she was killed by adventurers hundreds of years ago. Let's go get easy loot.
I see a huge number of plot holes in this from the player's perspective. Why would there still be treasure there if adventurers were already there (they would have presumably taken the loot)? Why hadn't the bandits already gone and taken up this "easy loot," rather than still working as bandits? This should have tipped them off that something else was up.
As for the attitude, some players feel entitled to do things simply because they're "the good guys" (even if they aren't). This is a good example for the players that they can't do whatever they want without consequences. I'm going to assume that they are new players (probably younger), as this can be a more common problem for them. I'd explain that you can't always win, but this wasn't that bad a loss, as they survived (for now) and have a chance to correct things.
I see a huge number of plot holes in this from the player's perspective. Why would there still be treasure there if adventurers were already there (they would have presumably taken the loot)? Why hadn't the bandits already gone and taken up this "easy loot," rather than still working as bandits? This should have tipped them off that something else was up.
As for the attitude, some players feel entitled to do things simply because they're "the good guys" (even if they aren't). This is a good example for the players that they can't do whatever they want without consequences. I'm going to assume that they are new players (probably younger), as this can be a more common problem for them. I'd explain that you can't always win, but this wasn't that bad a loss, as they survived (for now) and have a chance to correct things.