The past perfect is used because it's an event prior to his not wanting to go back. He didn't want to go back because last night had been brutal.

The answer to this, I think, lies more within the passage itself. The whole passage is in past tense. For example, "I didn't want to go back" as opposed to "I don't want to go back". In a passage where everything was already in past tense, "last night was..." just doesn't sound right to a lot of people. It blends in. Even though you specify that it was last night, the tense matches with everything around it and it sounds as if it's at the same time as everything going on around it. Therefore, to clarify that this is indeed in the past, it uses past perfect tense ("had been") instead of just past tense ("was"). If the rest of the passage was in present tense, you would only need past tense instead of past perfect. Does this make sense?

Using "was" in that sentence would have been better English grammar.