Bain (and their horrible ilk) put up a tiny amount of capital to secure huge loans. They used those loans to buy the company, taking it private. Once it is private they can do whatever they like with it; they pay themselves back for the loans by loading the company up with debt (and then some to ensure an immediate profit). They cut everything to the bone, fire as many people as possible, then spin the husk back out as a public company (or offload it to some other sucker).

Bain effectively risked nothing to make a killing and ensure Toys-R-Us would eventually file for bankruptcy due to an unsustainable debt load. Banks are happy to loan Bain money for these schemes because Bain makes a killing on them and private equity almost always finds a sucker dumb enough to buy the zombie company off them. (Every once in a while the zombie goes south so fast private equity is left holding the bag).

For loans to the zombie company the terms are punishing enough the bank only needs the zombie to survive for a limited amount of time.

The part I don't understand yet is who is the dumb money buying shares in these hollowed-out debt-laden husks from the private capital vultures? It almost always ends up badly. Even when it doesn't the upside isn't great.

FWIW Guitar Center is almost certainly headed for the same end and for the same reason.