Generally people challenge such concepts because they find themselves challenged and insecure. They fear that they can't commit to or they won't be able to live to your philosophy through out their life. So they want to disprove your stand.

My stand in the examples you provided would be similar to you. But instead of being apologetic later, the guiding force for my action would be compassion and love. And at times, you need to get angry with or even hurt some one out of love and compassion. Compassionate parents scold their children not for the sake of the fun of scolding them but for their own good in the long run.
If you push someone out of the way of an oncoming car, it is also the result of compassion. So it's not just your actions but the intentions behind your action that play a major role.

And while vandalizing the window, I'd do it even if I can't afford to pay off for damage done by me, later. Because, I'm being compassionate to myself too. Here guiding force is not to save myself from death due to any selfish interest but if I feel my life is more important to the humanity & all others than the life of this window-pane, I should choose the better option in the larger interest of everyone. While taking this decision, I ain't centred on myself but on the larger perspective of whole universe while judging the situation impartially. And I'm offering an opportunity to the owner of that premises to become compassionate towards myself. Asking for apology is just out of courtesy. In my heart, I'd empathize with the owner for his/her loss.

In such situations, philosophies of Lord Mahavira or Lord Buddha take the stand of supreme sacrifice. They preach not to fear even death to defend your principle of non-violence. And the vibration of non-violence is so contagious that there are stories that they were not harmed even by wild beasts like lion, venomous snakes and serial killers when encountering them in jungle.
To each his own.