Floor vs. child

I told the mini-humans to clean their plates from the table. Mini-human #1 (we'll call her mh1) dropped her paper plate in the trash while mh2 had a plate to put in the sink...Instead of mh2 going to the sink and dropping it in, she made a detour to hit her sister in the head with the plate before dropping it in the sink. That really pissed me off....am I a bad parent for being upset over spilled crumbs instead of being upset because my kids like to beat the hell out of eachother?