The charges against Daybell appear to be related to concealing bodies, not for murder. Maybe they offered Vallow something like avoiding the death penalty to reveal where the kids were so they could be laid to rest and the family be able to grieve for them properly. But I don't think investigators were offering her lesser charges to get Daybell.
In their divorce documents, Vallow's ex had said something about Lori believing she was a resurrected god. And one of the family members has shared she believed at some point the kids had been turned into...zombies? Possessed by demons? It was something along the lines of believing that they no longer had souls due to evil forces fighting against her. It seemed like this discovery was only a matter of time as the investigators seemed to be tracking Vallow and Daybell but from a distance after they were found in Hawaii but otherwise not rushing to ensure they could adequately build the case. That suggests at some point they weren't acting to find and save the kids. Who knows what they believed with good reason but lacked enough evidence to press charges. It doesn't seem like they viewed the kids as being alive and at risk for some time, though.
Regardless, it's a terrible series of events. You have a couple of people who believe God gave them a red pill that revealed they were central to the end time narrative, chosen and special, and essentially Neos to our "matrix". Offered that chance to be Neo on the one hand, or one of billions of other insignificant coppertops, they took the red pill and started seeing events around them as significant in ways only they could understand. Now, two kids are dead plus at least three adults quite possibly removed from interfering with their important, unique, divinely bestowed purpose.
I'll be blunt about why I think that while this may be more extreme as an example, it is built on common escapist religious themes that I find revolting. How many times has a believer told non-believers they think life without God or religion would be meaningless? That people who have left religion behind must be miserable or live shallow, empty lives? That comes from red pill mentalities. You don't think there's a higher purpose, a divine plan, that you are special in some way? That's blue pill thinking for coppertops. Red pill thinkers have seen the matrix for what it is, hate the matrix, and long for Zion. Chosing to reject God belief is choosing to take the blue pill, to be one of billions of insignificant people whose lives have no meaning "in the grand scheme of it all". It's not enough to create meaning in a life with friends and family, a career and a life lived well. Or lived, period. That's depressing. So, while Daybell and Vallow are on the far end of the spectrum, the spectrum itself is partially to blame for all the Daybells and Vallows becoming what they become.