Its just my belief, but I think civilians will willingly walk into fema camps at first, most people would obey a draft or volunteer in the right situation, and that gang leaders turned war lord will meet their end under heavy artillery bombardment.

Very good thinking. I have seen something like that.
You see there is no situation like real “absence of law”, problem is that when SHTF people who will try (or succeed) to gain power in some area (let s say city or state…) will have different ideas about law and human rights and similar things.

Biggest problem is that most of the ordinary folks want to have someone to rule over them, they want to trust in law, they want to trust in system, to be part of the system.
Most of the deaths in beginning of SHTF scenario is about fact that people OBEY to the wrong guys, with wrong orders.
And I have seen people WILLINGLY going to the private prisons, work camps, or simply to be executed just because the believed in orders from the guys who looked or say that they are law.

Real vacuum is there only if you look from distance, if you look closer you ll see that there are gonna be many groups (fractions, governments, militias, paramilitary or whatever name is gonna be) who are fighting for power and resources without any moral (ethics).

When SHTF most of the people will look for outside help, for someone to tell them what to do, to organize them, to help them.
All that sounds good, but better idea is to be organized and prepared before SHTF and to not trust nobody when SHTF.