If a massively powerful and knowledgeable AI was created by accident, it would have nothing to gain by being malevolent.

The point is that, if you create an superintelligent AI that isn't very, very highly optimized for helping us, it is very likely that it will hurt us at some point.

Or as the saying goes:

The AI does not hate you, nor does it love you, but you are made out of atoms which it can use for something else.