The Trump campaign said he "has a fiduciary responsibility to his business, his family and his employees to pay no more tax than legally required." While that's ludicrous - one might as well say he has a fiduciary responsibility to all of them not to over-spend on personal helicopter rides - I would agree that he didn't have a responsibility of any kind to pay more than was legally required. But as I discuss in this article, the legal defensibility of one's positions doesn't put an end to all ethical issues, especially if one is planning aggressively in "unintended" ways, relying on legal uncertainty and the audit lottery, etc. (Yes, you can be audited every year and yet be playing the "audit lottery" - the relevant question is whether the auditors detect a questionable tax position with sufficient acuity to subject it to proper challenge based on a decent understanding of the underlying facts.)
Still, I think the bigger takeaway from this is that he's a bad businessman (other than when selling his name as a brand), and that he wanted to try to hide all this from public view, rather than getting it out early and moving on.
The revelation of likely use of NOLs to zero out tax liability helps one to refine the questions associated with the Trump Foundation diversion-of-income scam. To the extent he still had NOLs, they would have sheltered the diverted payments, but this might have had the effect of using up the NOLs faster (depending on what was left, how fast they were expiring, his ability to "freshen them up," as it's called, etc.).
Insofar as he would have had offsetting income inclusions and charitable deductions - seemingly true for some but not all of the Trump Foundation stuff - if he was getting his income to zero anyway then he wouldn't have been able to claim the charitable deductions due to the AGI limit (30% for gifts to private foundations, 50% for most other cash gifts). But there is also a charitable deductions carryover - generally 5 years. Use of the carryover is itself, of course, subject to the annual percentage-of-AGI limits.
Might there be a scenario in which Trump's possibly illegal use of the Trump Foundation as a diversion of income piggybank might actually have been bad tax planning? Not impossible, but one would need particular numbers to make this scenario work out. Still, let's give it a try, if only for the fun (?) of it. Say that in Year X Trump has $1 million of expiring NOLs. He ends up with zero taxable income due to diversion of $1 million to the Foundation, so the NOLs expire unused. Say also that the Trump Foundation uses the $1 million in a way that would have given him a charitable deduction if done directly.
Now in the non-diversion counterfactual, he has $1 million of income pre-NOLs, zero post-NOLs, and a $1 million charitable deduction that he can't use. But he also has a $1 million charitable contributions carryforward that he can use to that value over the next 5 years (albeit, subject to the 30 percent limit annually if it would have gone to the Foundation, 50% if it would have gone directly to the relevant charity).
This is similar to "freshening up" NOLs, although the charitable contributions carryforward is less valuable than an NOL (shorter duration, AGI limit). We know far too little about Trump's tax returns to know if this scenario would ever have applied, but he certainly seems dumb enough (and poorly advised enough, since he doesn't seem to want good but independent people working for him, plus you can't count on him to pay you) that one shouldn't rule it out.
A broader point, though, is that just because he uses an illegal scam doesn't mean it's actually helping him. It's just the way he operates.