The primary issue with the Roko matter wasn't as much that an AI might actually do but that the relevant memes could cause some degree of stress in neurotic individuals.

The original reasons given:

Meanwhile I'm banning this post so that it doesn't (a) give people horrible nightmares and (b) give distant superintelligences a motive to follow through on blackmail against people dumb enough to think about them in sufficient detail, though, thankfully, I doubt anyone dumb enough to do this knows the sufficient detail. (I'm not sure I know the sufficient detail.)

...and further:

For those who have no idea why I'm using capital letters for something that just sounds like a random crazy idea, and worry that it means I'm as crazy as Roko, the gist of it was that he just did something that potentially gives superintelligences an increased motive to do extremely evil things in an attempt to blackmail us. It is the sort of thing you want to be EXTREMELY CONSERVATIVE about NOT DOING.

(emphasis mine)