Look, I get why you have personal issues with this based on your background. It's called a trigger. When someone has a trigger due to some traumatic experience from their past, they have to avoid certain situations. I get that. But think about this. Maybe yours is not the most objective viewpoint on this subject and maybe it never will be. I don't know if I can ever say something that will get you, specifically, to look at the subject objectively and rationally. Every time you bring up this subject, and you do a lot, it feels like an energy vortex that's going to suck up my time and energy pointlessly. Each time, I know I'll have to repeat everything we've already discussed (a couple of years now?). All progress seems to be lost each time and we have to start ALL OVER AGAIN as if you're still a total noob to this subject.
That perfect example of a straw man that you just posted is exactly what I'm talking about. I feel like I'm back to explaining the difference between consensual activity and aggressive violence. And you picked such an extreme case--I too would seriously question such a practice even if it were completely consensual (and not involving pedophilia!). Making this comparison is offensive in the same way that comparing homosexuality to bestiality as if the two are in any way comparable.
As far as this horror story that you're fabricating, as if it were some common occurrence that people ignore crimes because it might be consensual play, most BDSM people just aren't that stupid. And the ones who are deserve whatever happens. There have been cases of people walking in on private BDSM sessions and it typically does result in, at a minimum, a lot of embarrassment for all parties involved as everyone has to confirm that it is, in fact, completely consensual. I would expect a criminal who's trying to abduct or rape someone to maybe try to pass it off as a consensual activity. This is why it's such a straw man. People into such activities are well aware of how important it is to establish consent clearly beforehand and anyone who doesn't get that would be ostracized by the broader BDSM community and deserves to be perceived as a criminal.
Such an investigation by bystanders is totally appropriate and the depth of the investigation should be proportionate to the severity of the situation. If someone is bruised, naked, and in chains in a basement, that justifies a pretty extensive investigation. Hell, even if the alleged victim claims it's consensual, if they've been abducted involuntarily for a long enough time, they may be suffering Stockholm syndrome. If I knew someone who wanted to engage in a lifestyle that extreme (dom or sub), I'd first try to talk them out of it because it sounds really unhealthy. Failing that, I would say to make a video explaining exactly what they're consenting to that can somehow be dated--maybe uploaded as a private YouTube video, and tell several friends who can corroborate the story BEFORE they turn themselves over to the person. That is, if they care at all about their "master" not going to prison.
I don't want to imply that I don't care about convincing you what's wrong with what you're doing. I just don't think this conversation with you can ever be productive.