You said it yourself. I do not think you should doubt if it was insincere. This in itself means that you would not be guiltless if you hurt another being. You are not going crazy. You did something bad, you can now learn from it. Now you know that when you allow yourself to get that mad, you can do harmful things. Don't do it again. If you can not consciously make the decision to not harm another being or thing while that angry then do not allow yourself to get that angry. Walk away before you get there. You are fortunate to have learned something about yourself and be able to learn from it. Take advantage of that.