a little unconventional, i know, but for the sake of secrecy please humour me! i'll get back to you in the next few days if you've been added to the suspect list or have gotten the culprit slot.
IDEAS FOR MOTIVE, IF YOU'RE STUMPED:
shadowclan/thunderclan cat that saw rushwater's strategy as a threat to their clan.
riverclanner who feared his authoritarian beliefs.
rushwater found out/discovered a great secret of theirs.
anything! get creative, have fun with it! just be open for discussion.