There are 3 intellectual errors that intelligent and moral people regularly make.
The first is to assume that others are bound by the same moral codes as you are. This is simply not true, and the entire history of humanity proves otherwise, and yet – many good people still cannot help but continue to make that assumption, because they find the alternative ‘unimaginable’ – well, you don’t have to imagine it, because it’s real.
The second is to assume that anybody of intelligence would, by definition, also be moral. Again, this is completely false, and whereas your intellect might have positioned your moral positions, for others it is nothing more than a tool to achieve whatever their goals are, moral or otherwise – again – I won’t bother with historical examples, but the most cursory of historical overviews will confirm that this is a completely invalid assumption.
The third error is that ethics are derived from morality, and therefore that all ethical positions are therefore morally ‘correct’ – that is, again, completely false, ethics are about protocol more than they are about morality, and often come into direct conflict with morality, for a case study of the differences between ethics and morality, I suggest you watch the entire 8th season of The Practice and focus on the character of Alan Shore, a man who betrayed ethics on a regular basis, but did so in the service of morality.
It is easy to make all three of those mistakes, and they are mistakes most often made by those with both intelligence and kindness, but they are deadly mistakes to make, and can, literally, cost lives.