It would have made very little difference had Hitler held open doors and helped old ladies cross the street. It makes no difference to us whether Karla Homolka is big on recycling, or whether George W. Bush is a good friend.
These people made decisions with large ethical impacts that drown out the significance of their “ordinary niceness.”
The same might not be true of the reverse. If someone that does great things for humanity comes off as an asshole, I think that might really change how the public feels about that person. Philosophically, however, that makes as much sense as growing sympathetic to Hitler for helping old ladies across the street.
Whether or not my intuition is true, there’s a very real distinction to be made between grand ethical behaviour and ordinary niceness.
Ordinary niceness refers to common sense gestures used to determine whether or not someone is a good person or not. A nice person is someone that holds open doors for others, helps old ladies across the street, treats their pet nicely, makes other people feel comfortable, keeps promises, is generous, etc. None of these acts have very large impacts on the world in the grand scheme of things but they’re typically used as indicators of ethical grounding. This is probably because we can be pretty certain in saying these acts have small positive effects on the world, almost no matter what our ethical views.
The best ways to do good in the world have very little to do with this kind of niceness. Actions with mass positive effects usually come from making political or technological progress. It can be a lot more difficult to determine whether these acts are having positive effects at all because of how their utility fluctuates in accordance with the contents of our ethical principles and various complex real-world factors.
Considering well-developed ethical principles don’t make people more likely to act nicely, we should consider the relative moral importance of niceness compared to the embodying of grand ethical principles. (Some examples of embodying grand ethical principles: not eating meat, choosing a “good” career, protesting against “bad” causes, voting for the “right” party.)
Ordinary niceness is arguably close to irrelevant in the grand scheme of things. Maybe an effective altruist holding doors open for people is changing his or her overall impact no more than Hitler changes his impact by holding doors open. I can think of two reasons for ordinary niceness: (1) it has a reliable positive impact that accumulates over the course of a lifetime and (2) it is a good PR move, especially for people trying to convince others that they’re in the business of maximizing the world’s wellbeing.