Question in title, situation is you have identified a GPL violation for which the authors refuse to comply and release source.
The authors reside in a country which is known for its rampant violations and theft of intellectual property.
Is it equally unethical to reverse engineer their modified software and release it as open source? Two wrongs don't make a right etc.
You're not "stealing" source code, you're deducing operational concepts and then reimplementing it with your own work. That's perfectly fine in a lot of, if not most cases.
(Legal exceptions: your country forbids reverse engineering, or the software is patented [= in both cases, your legal system sucks]. But those are legal arguments, not moral.)