You are quite right about the "study in frustration" element, and someone else also pointed it out to me. However, this could be read as the comical counterpart to what would have otherwise been a really too grim depiction of lust in action ("the expense of spirit in a waste of shame") for a play that must have a comic side. I could as easily see a drugs story in which the addict is really villainous and obsessed but always ends up messing up - for the same reason as he is the villain. After all, to a very large extent the Don lays the groundwork for his own discomfitures. He lets in anyone into his party, so that Don Ottavio can actually bear witness to his villainy and react to it; and he comes back to his own house to party, regardless of the fact that he should now be aware that Don Ottavio and his friends want him jailed or dead.