How does attacking a city bus help them other than some misguided sense of revenge?

In the context of your thought experiment, it would be like attacking the family of the people attacking your family, when their family was not involved in their crimes. As an outside observer, why should I actively support the person in your thought experiment if the crime against his family turns him into just as much of a monster as those who attacked him first, someone who is willing to the exact same tactics as those he hate? I would just feel bad that they were attacked on the first place, recognise it as the catalyst for them abandoning their own ethics which is a tragedy in itself, and not want to take sides in what is currently happening between them.

If you want me to take the side of one party in any general situation, then I will personally want them to be materially better than the other side. Otherwise I'm defaulting to taking no sides.