Then why did it come up as "not guilty" for her, then? If she placed the killing blow, that would be enough to prove her guilty.

[ ... ]

You don't have to be in the room with someone to kill them.

The deception in all this came from making Tuuri really believe she did this. Neither us, nor her, know what happened between her lashing out at Kon in the ice cream parlour, and her over his body in the biome. That's a massive window of opportunity for the culprit to do literally anything.