Having cast-off blood on her shirt doesn't mean she wielded the knife. It only means she was in close proximity to the attack. We already know that since she was on the couch.
The analysis of the blood drops shows that they were deposited from overhead. How did they get on the back of Darlie's nightshirt if she were lying face up on the sofa,AS she claims. . IMO the drops were deposited when Darlie needed more force to get the knife to penetrate the boys bodies. There were shallow stab wounds in addition to the penetrating ones. She was kneeling on the floor and to get more force for a deep stab, had to raise the knife higher. Both boys blood dripped off that knife when she raised the knife to shoulder, or higher, height.
First, there is absolutely no way to know if at some point she wasn't laying on her side. People who are unconscious typically can't recall every position they were in prior to waking up. She woke up on her back. That doesn't mean she was in that position the entire time.
Secondly, that is not what Bevel testified to. There are five stains total. The stain on her back couldn't be directionally determined. It could have come from up to down or down to up. He couldn't make a determination either way. He decided that the stain could be consistent with that spot of blood flying over her shoulder and landing on her back with an up to down trajectory. But the stain could also be consistent with her laying on her side and the drop landing on her back with a down to up trajectory.
The other four stains were all a mixture of her blood with the boys. Three had a down to up trajectory, in his opinion. These three stains Bevel thinks are consistent with spatter, not cast-off, as the heel of her hand impacted blood and spattered up. The deepest wound to either boy was a little over five inches. The knife blade was a little over seven inches. That leaves a two inch gap where the heel of her hand could impact anything.
The fourth had an up to down trajectory, again, his opinion. This particular drop of blood supposedly flew straight up into the air, hit it's highest point then came straight back down. Of course, cast-off typically flies off of a knife tip traveling away from the perpetrator. It normally casts off on to walls, furniture, ceilings, other people who might be in the room, etc. This drop must have boomeranged back on to her.
The four stains on the front are particularly important because each one contains Darlie's blood along with either boy. When Bevel was told this was inconsistent with Darlie stabbing herself last Bevel testified that four drops of blood, from the boys, landed on the shirt. Then, in a separate event, four drops of Darlie's blood landed directly on top of the other four drops. That defies logic on multiple levels unless someone is going to tell me blood drops are magnetic.