I'm not sure if I should recommend this tactic, but you could design a legitimately evil character that acts as a mirror to show them how their actions look from the outside.

One way to accomplish this is designing a LE character who will do terrible things if it benefits him; but he looks down on the party's random acts of pointless cruelty, laziness, arbitrariness, and other less than redeeming qualities.

This is a terrible idea for a lot of reasons, but another is designing a CE character who absolutely loves the party's arbitrary pointless cruelty. Everyone will hate this character, but that's the whole point. Once they hate the character and all that he represents, there's a chance they'll stop acting arbitrary and cruel.