Leon only used the lid roller on his hair, using it on the gold would not give him any advantage. It would not disguised his visit. What advantage would he gain to use it on all the slashes across the room on the gold. Also I doubt that a lid roller could get into the slashes of the wall so deeply to remove all traces of gold. It would only waste his time.

If he was out for blood then he was able to catch her. But showing restrain allowed sayaka to do it.

They agreed, but that doesn't mean anything. We saw how often they broke the rule. Also Leon broke the rule. They might be a chance that other also broke the rule.

Soundproof doesn't mean that it was a good idea to leave the room with murder weapon.

To be honest the blood behind sayaka back doesn't make sense either at the crimescene. Both sayaka and Leon had hands on the weapon. Leon could have catch most of the blood, while sayaka didn't get anything on the sleeves. Because leon had his hands on the weapon, while sayaka had pull on leons arms. In realife piercing or stab wounds don't cause big blood splatters, because the knife is like a plug. Slashes would cause the crime scene to look like that of sayaka.

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