They did this because:
1) they were very very offended by those statues
2) they had the power and they didn't really care how people of different beliefs feel about those statues
3) because (in their opinion) there were some "Truths", dear to them, that other people do not have right to question, much less deny
4) Actually, other people weren't 'real' people to them, so when they were done with statues they turned their attention to blowing up people.
My point, of course, is that current situation in US is Trump's fault, blame is certainly NOT on both sides and if you perceive ALL human beings as human beings with rights and feelings then you are a nazi-sympathiser.
Have a nice day and try not to think too much.
