Are you kidding me?

Tonight I had school, so the boys had to go to Grandma and Grandpa's house to be babysat. The twist tonight was that it was Gavin's open house at school.
At about 4:15 I fed them hot dogs and apples so they wouldn't be hungry at school. After they were finished I had Quinten go to his room to put his shoes on and sent Gavin to the bathroom to get his teeth brushed.
After Quinten was done, we went over to the bathroom, where Gavin had prepared Quinten's toothbrush for him.
I sat Quinten on the counter and started brushing, but the reaction wasn't normal. He had an uncomfortable, sour face. Then Gavin says, "How does it taste Quint?". I didn't catch it right away and then the light turned on. I took the toothbrush out of Quinten's mouth, looked at Gavin and proceeded to ask what he had done to it. Gavin actually kept a straight face while he told me, "Nothing". I asked Quinten if the toothpaste tasted funny and he said that it did. I looked back at Gavin and turned up the interrogation heat. He finally told me that he was trying to clean Quinten's toothbrush with hand sanitizer. I looked at him and yelled, "Are you kidding me?". His expression told me he knew what was next. He tried to explain that it was for the betterment of his brother's dental hygiene, but none of which I wanted to hear.
I grabbed his arm, which was covering his bottom and told him he better move it, or the results would be detrimental. He refused. I finally wiggled that arm out of the way, only to be greeted with another arm. I finally grabbed him by both arms, took him to the hallway, placed him face down on the carpet, sat on his legs so he couldn't kick me, and gave him a really good whooping.
He was then sent to his room for 5 minutes while I regained my composure. If you hadn't noticed, I lost it a minute earlier. Gavin was ordered to apologize and to do so in a very nice way!
We then went to my parents, which I couldn't have been more happy for by this time. I explained to my Mom what happened and she called him a name, which Mennonite's don't type on blogs. I told her not to check his bottom for a couple hours, until the welts go away.
It always feels good to vent to another Mother, who understands. She didn't look at me crazy or argue with my reaction. She agreed! That was exactly what I needed because even when your kids act like the devil's spawn, you still feel guitly for disciplining them. The reasoning behind this reaction is still unclear to me.