Terry Hobbs was known to have disciplined his child - often severely. His aunts have provided declarations as to that fact as part of the Pasdar case. (
Declaration of Sheila Hicks Muse)(
Declaration of Jo Lynn McCaughey) So, Steven could have been the one being disciplined. If TH found the boys playing in a manhole when they had been told not to do so, IMO it is entirely possible, given his past and subsequent violent behavior wrt to his son and others, that TH could have started disciplining Steven. It went too far, or maybe Steven accidentally was knocked against a hard surface, like a manhole cover, and Steven was unconscious.
If TH thought Steven was dead, then he would have wanted to eliminate the other witnesses to his deed - Christopher and Michael. What led so many astray in the beginning was the supposition that, since Christopher appeared to be the most severely injured, if one child was the target of the attacks, it would have been him. However, when later evidence of animal predation was presented, and Christopher's most graphic injury (the degloving) had another explanation, the injuries to Steven then became the most severe, casting suspicion on TH. Then, when mtDNA matching TH (and 1.5% of the population, to be sure, but how many of them stated that they were in those woods on May 5 -6, 1993?) was found
in the ligature binding Michael Moore, not his step son, Steven, IMO, the circumstantial evidence against TH reached a tipping point. Again, IMO, the evidence against Hobbs is much stronger than that against Damien, Jason and Jessie.