What happens when you attack DeFi?

Background: DeFi (decentralized finance) is the movement on the Ethereum Network to build and use finance building blocks to allow for any kind of financial instrument needed, in a trust-less, decentralized way.

Using these building blocks, this weekend someone borrowed $2.7M USD to make $350,000 in a single transaction (that is, atomically - the loan was repaid as part of the same transaction) by exploiting some weaknesses in at least one of those building blocks. This is one of the better explanations of how that "attack" worked, plus some cheerleading about why it's actually a good thing.