By principle of charity, I interpret Marshall as saying not that rationalists can't be kind, but that rationalism alone doesn't make you kind. Judging by my informal torture vs. pie experiments, I find this to be true. Rationality is necessary but not sufficient for a friendly world. We also need people who value the right kind of things. Rationality can help clarify and amplify morality, but it's got to start from pre-rational sources. Until further research is done, I suggest making everyone watch a lot of Thundercats and seeing whether that helps :)

Of course, like with every use of the principle of charity, I might just be reading too much into a statement that really was stupid.