I mean, it’s one of the things we do at my lab, but in this case I just don’t see a likely connection, plus the DW is a vast, unruly space, which doesn't make life easy for generic crawlers, and even purpose-built Tor crawlers are tricky to design, requiring dedicated protocols and management, and are still often unreliable and finicky in terms of weeding and processing.
Short answer: if there really is relevant stuff to find there then I fully expect the FBI to find it.
I've never heard of any DV support group using the dark web.
There are actually a number of groups who have looked to build safe spaces there, including some with specific focuses on sex worker safety and outreach, etc., but even with instruction and a purpose-built browser many don't find that navigating is easy, and there are plenty of other, more straightforward ways of setting up and managing encrypted groups for support that would not leave breadcrumbs for spouses or partners to follow easily.
That said, I just don't see BM as a person who is going to devote himself to learning about ways to track and trap, then set it up, monitor it, and devise a murder on the basis of what he finds. Anyone with a teen can tell you that even with a dedicated app it's a huge PiTA, and requires a ton of your time. Maybe he did this sort of thing and then confronted SM with what he found, leading to a violent attack, but it doesn't seem the most likely scenario to me.
ITA with That Guy From Mass and others who have painstakingly laid out the timeline alongside what we know about BM: this is a controlling, though perhaps generally non-violent partner, who either was found out, or found out something that then escalated into violence, necessitating the messy cleanup and messier basket of stories, timelines and interviews we have now. Maybe there's steroids and/ or AN Other person involved, but every gym I've ever worked out of had a guy or more who was reputed to be ably to supply juice, and there's a gazillion message boards on the regular web where that stuff can (supposedly) be procured.
I trust that LE have been examining these as we have here, with all the additional tools they can bring to bear, and that we'll see an arrest at some point and hopefully have a clearer picture of SM's story and location.
If something like a "I need a hitman" post does turn up, on the DW or anywhere, I don't expect it to be much more than another shiny nail in the proverbial coffin. IMO.