When you are a failure

How long shall I keep on pretending that all is OK and that I have a normal child when there is clearly something wrong?
Went to play group today and within five minutes, Leo had managed to throw him self on the floor, kick and screaming, because one of the teacher said 'hello', he had snatched a toy out of a 4 months old hand, and pushed another girl three times, whilst growling at her.
Five minutes.
So I packed up and left.
I can't do the softly-softly voice anymore, negotiate and tell him, yet again, that we don't push, let's play nicely, and share.
Can't do it.
So I guess I am going to be pretty isolated from now on, and Leo will just become even more of a monster.
There is nothing that can make you feel like you've failed so utterly miserably as when your child as acting out in all the ways you thought you had raised him not to.

I've had friends who've told me that they stopped going to certain playgrounds or play groups, because their child couldn't behave. I always thought they gave in too easily. I thought: don't let your child run the show, and dictate the terms. It's your life, your day too, show him that he can't spoil it for you.
I know exactly what they are talking about now.