I don't think Kevin's evil was treated as a mystery to be solved; if so, the film never answered the question. And I don't think the murders and his evil are synonymous, either. We know almost from the very beginning that he kills people, but what we don't know is why. Watching him kill doesn't help answer that, and in a way makes it more perplexing. The film is much less interested in his motivations than it is in his relationship to his mother, which I think the final scene demonstrates. Kevin has no answer for why he did it, but that's not the crucial moment; rather, the moment they embrace is the moment the film has been building towards. It is here that the character parallels are most emphatic, and yet still remain ambiguous. Both forever changed their own lives because of each other, and both do not understand why. They find some comfort in realizing their mutual ignorance and suffering, but the audience must decide on their own whether this constitutes love, and if so, if it is any more honest or perverted than the relationships they had with the rest of the family and the outside world. I wouldn't characterize that kind of ambiguity as banal.