Generally I think that, because we can only ever observe one timeline, people who do that kind of “good” need to be content with the fact that their legacy will likely be negative.
It may be that you believe that you are working for the greatest achievable good within a system, but you can never truly know what you might have accomplished by opposing a system outright, nor what kind of change you may quash by aligning yourself with the system.
I think it’s not an unreasonable position for a person to take, but I also don’t see how it can be seen as ethical, as it’s ethical only in contrast to a hypothetical that we can’t be sure of.
But to address your OP more specifically, the ethical line between “peaceful execution” and “painful execution” is invisible compared to the ethical line between “execution” and “no execution”. While I’d have my preferences as to which way I’d like to be murdered, their crimes are fundamentally equal in my book.