The whole left and right thing is part of the problem. We're herding animals and we inherently see people not in our herd as sub-human and inferior. This makes crimes like the terrorist attack happen. It's also behind a lot of anti-social behavior. If you blanketly label 50% of the population as your ideological enemies, even when there's likely plenty of overlap in beliefs then that's a big problem.

We have to somehow see each other as human beings, that's why it's important to focus on the victims and not the terrorist. You can have multiple countries living together in relative harmony. It's not hard, you just need empathy and respect from all sides.