There’s no realistic scenario for one to have evidence that MM is dead without the remains being located or a tape of MM being killed and/or her corpse being buried in an undisclosed location (the perpetrator’s face was either covered or did not appear on camera).
If you catch a serial rapist and killer who lived right next door from the resort MM was last seen and the remains of 20 other 3 year old girls are dug from the backyard of this property but none of that is a match to MM’s DNA, that still wouldn’t be evidence that MM is dead (or killed by this fella). You could convict him on those 20 other cases, but not on MM.
Odds are this creep also killed MM but buried the body elsewhere. But it could be that MM was a victim of other creeps that are in the radio of any town or city, or that her demise was not related to an outsider as well. Everything is possible. The problem with locking in on a ‘boogieman’ is to forget that other boogiemen are also out there.
There are many cases where serial killers were caught and tried to take credit for other high-profile crimes for the sake of ‘rising their public status’ (they knew they would be locked away for life, they had nothing else to lose). But if they can’t provide some independent information as in something LE never shared with the public and only the killer could know or, better yet, lead them to the remains, then the confession shouldn't be deemed credible.
Because even those who are 100% behind the abduction theory wouldn't want the "wrong fellow" to be found guilty for MM's demise, right? Even if for the sake of some other psychopath walking free and making other victims in the community? Boiling every discussion regarding CB to "he has a criminal history and pattern of offending", IMO, is what leads to most pseudo debates: it prevents the discussion from going beyond the established behaviors of this person's criminal history.