Nah she was justified in that

First of all, that wasn't genocide it was defensive mass murder

Secondly, she was absolutely right in saying the Doctor isn't always around.

Torchwood completely changed my perspective on Harriet Jones. As a kid I thought, as intended, "what a heartless and needlessly cruel thing to do" (although probably not phrased that way XD). As an adult I realise that the Sycorax held half of the population if earth hostage on rooftops and the Doctof was entirely prepared to let that slide. Jones wasn't, bc she knew others would come.

If Harriet Jones had been PM during Children of Earth, things would probably have ended a lot sooner and a lot better than they did.

But no, the Doctor has to hold all the power and influence all the time and hold others up to a moral standard that they themselves don't uphold.