Step 1 for being an ethical patent troll would be having enough assets that you aren't just a judgement proof vehicle for lawsuits, but there's no reason that isn't possible. A diverse portfolio of legitimate patents, a healthy bank account, and voila.
There are a variety of reasons why forming a "real company" that produces products might not be practical. For example you might be in a field where the startup costs for a competitive company is in the billions (silicon manufacturing), or that is a natural monopoly already monopolized by one or two big companies (operating systems). Your competitors also have patents on things that you would need to be competitive and for whatever reason you aren't willing to license them.
Or really you might just not be well suited to running a company, bad at managing people or whatever, and if patents are legitimate it seems like they are legitimate regardless of whether or not you want to start a company.
(PS. I'm generally against patents, and strongly against software patents, but that's neither here nor there on whether or not patent trolls are legitimate under the assumption that patents are)
