Hitler, anyone?[0] (sorry Godwin)
[0]: https://en.wikipedia.org/wiki/German_presidential_election,_...
We did commit all three of those atrocities you mentioned. We are imperfect. We also did the best we could the time. We have also improved since then and attempt to actively prevent nuclear proliferation. We have developed bombed that can destroy a single building making arms and leave the hospital across the street untouched. Consider the 1991 gulf war. It had the lowest rate of civilian casualties of any war up until then. We have never set out with the goal of killing enemy civilians out of some direct malice, sometimes we did it because we saw an existential and no other choice.
We blew up cities because we thought it was the best way to destroy factories making arms and we mistakenly thought it would scare the enemy into defeat. Our leaders didn't understand the psychology of our enemy. We have never demanded "death of all infidels" or beheaded our enemies and used that in promotional material. ISIS is the ethical equal of a military of any modern world power.
Democracy is not an blank check that allows anything. Even some of the worst leaders in History were actually elected through regular, due process. And there is no principle that shows that Democracies don't engage in Mass-Murder of civilians (erm, Dresden? Hiroshima? Nagasaki? And many others)...