Everyone is entitled to a defense, but Simon Vickers' portrayal of events was completely inconsistent with the forensic evidence. The fatal injury sustained by Scarlett was clearly due to a knife, firmly gripped by her father, being pushed into her chest. This tragedy is heartbreaking for everyone involved, but that doesn't free Vickers from responsibility.
While the court clearly didn't see it this way, there's always a chance that Vickers's unusual portrayal of events is the truth. He and his daughter, whom he described as his "love," may really have mistakenly grabbed the wrong utensil when play fighting in the kitchen. It is clear that, irrespective of the sentencing, we shall never know for certain what really caused this family tragedy.