Your post contains a lot of contradictions.
First, private equity firms do invest a lot of their own money in the firms they buy (hence the name. They invest private money (that of their owners/shareholders) into equity by buying firms). If they didn't banks would be rightly suspicious of the deal since the private equity firm would get paid no matter what happened.
Second, because of this, it is in the interests of the private equity firm that the company do well. In theory, if the PE firm bought a controlling share (say 51%) they would be in a position to screw over the remaining 49%, for example by selling the firm's assets to some other company owned by the PE firm, at a discount rate. This would most likely be illegal. Apart from this mechanism, the interests of the PE firm are exactly the same as the remaining shareholders: to get as much money from the firm as possible.
Finally, if it is more profitable to sell a companies assets and fire all its workers, then this is a good thing. If you don't already believe this then I don't have space to prove it in this post, but the general idea is that labor and capital are just like any other commodity, and so a company whose share price today is less than the value of its in place assets, is like a giant inefficient machine that consumes capital and labor for what is effectively negative profit.