Not always ethical, but hard to pass up in most cases. I also plan to do what you suggested (the 401K to Roth thing, not the murder thing) but I do have a problem with someone paying a lawyer/lobbyist $1M to avoid paying $10M in taxes. However, I also disagree with the entire idea of the estate tax so it's hard for me to rip someone for trying to avoid it.
Like everything in life it's tough to find the line. Why is it right for me to look down on the guy collecting unemployment, disability, welfare, or food stamps if he truly needs them, while I intend to avoid paying tax after FIRE via the Roth ladder, just so I can retire earlier with a higher safety margin? Am I not turning my back on the most vulnerable in our society?
Why is it socially acceptable for a man to marry a woman, but until June 2013 it wasn't legal, and still isn't always socially acceptable for a man to marry a man? Why do I instantly "look down" upon a man with multiple wives before really thinking about why I do this? Maybe it is okay, and we as a society just haven't realized it yet.
A bit off topic there, but my point is that right and wrong, ethical vs. not ethical, is a very tough thing to call most of the time. In regards to the estate tax avoidance/evasion though, I don't like the advantage the ultra weathly have to manipulate the system, so I'm against the unfair advantage. I hope I anwered your question rather than creating two more.