I wonder what the deleted Roko comment was about

Are you talking about his -18 karma comment? It says:

Long post on eugenics, -1 points right now and lots of comments disagreeing. Looks like this is a political battle; I’ll skip actually reading it and note that these kinds of issues are not decided rationally but politically, EA is a left-wing movement so eugenics is axiomatically bad. From a right-wing point of view one can even see it as a good thing that the left is irrational about this kind of thing, it means that they will be late adopters of the technology and fall behind.