Based on publicly released information about the case, Yaron Segal seems a very unusual alleged potential child molester.
He negotiates directly with the parent (not circumventing the parent to access the child directly), not negotiating from a position of power, respect, or authority (hello Catholic priests), in a venue clearly labeled for this kind of activity, namely the internet chat rooms "childrapesex" and "childrapetorturebrutality" (not subverting some other-purposed venue, e.g., a church), negotiating with a mother who has joined these clearly labeled chat rooms on her own free will, and is explicitly clear to the mother (disgustingly so) about what he would like to do with the children, giving her complete information and control about whether to opt out or let this man access her children (to which she chose to allow access).
It might be argued that removing this kind of man from society prevents him from doing less "transparent" crimes in the future, but that seems to me prosecution against "pre-crime", acts not yet committed and a significant leap of speculation that they will actually happen (e.g., he would have some day become a priest, achieving a position capable of non-transparent crimes).
Unfortunately, due to homicide or suicide, the case will not go to trial. I wonder whether a defense may have been possible on the grounds that the mother gave informed consent, and whether it would have been legally precedent-setting that crossing state lines is illegal when the mother has given informed consent to visit.