Brendon did not target innocents. He didn't attack teenagers at a pop concert or families enjoying a night out on a public promenade. He struck a highly effective blow against part of the political machine that is still actively engaged in attacking his people and attempting to eradicate them. If you don't believe violence is a legitimate way of resisting invasion, if you don't think that making war on those making war on you is permissible, that's your prerogative, but your opinion is both ahistorical and irrelevant.
The fact is that Brendon Tarrant not only gave up his freedom to strike back at the invaders who are actively seeking to destroy your nation and your people, but he did so alone, and in the full knowledge that he would be hated for it by many of the very people he sought to save.
You may recall that someone once said something about the quality of the love that such a self-sacrifice requires. Can you honestly say that it was nothing but simple hatred that inspired him?
Those who are not religious cannot fathom that kind of love, which is why they simply deem him mad, and a monster, and try to avoid thinking about the future. I don't expect you to simply accept my perspective, but it might give you some food for further thought. While he may have done a "terrible thing", it is far more terrible that he was put into a position where he felt the need to do it in the first place.
In any event, should the West, and Oz, survive the ongoing clash of civilizations, Brendon will be considered its first hero. And if it does not, well, then Brendon will be regarded in much the same way that Robert E. Lee, Jefferson Davis, and P.G.T. Beauregard are presently regarded in New Orleans, America, as an evil monster who was "on the wrong side of humanity."
At the end of the day, he was simply practising politics by other means in a world where democracy is only a one-way train ride to hell and we are in a state of war by all definitions. If every man had his dedication and determination, we'd be living in a much better place. And for that alone, I salute him.