This was easy from the start.
Also, putting his own t-shirt to frame the other guy was incredibly risky because of the high probability of it containing his own DNA.
@Colomopri
That would be my guess as well. Kaoruko offering herself to fix the garden was also suspicious because it seems the garden has been tampered and she would like to fix it before anyone notices. It could be related to the trick of Sakura's murder.