The Staircase documentary on Netflix was very interesting but I don't know what to make of it. I read some theories online as to why Michael was guilty and here is part of one below.
At some point, he realized how devastating - and mortal - her wounds were, and he let her bleed out long before finally calling 911 at 2:40am. Problem is, the autopsy results (red neurons in the brain) and testimony from both defense and prosecution expert witnesses show that Kathleen was dead at least 30 minutes to 2 hours before Mike called 911 at 2:40 am when he said she was 'still breathing'.
I believe while they were at the base of the stairs trying to assess the situation (or possibly struggling in a panic), Mike realized she wasn't far off from losing consciousness (long before ever calling 911) and actually led her up 4-5 stairs, then purposely let her go (or shoved her with his foot, which explains his bloody shoe print on the back of her pants legs) and allowed her to fall down the few steps which ultimately rendered her unconscious and incapacitated, which is ultimately what he wanted at this point.