OK, so you believe humanity will destroy your species in four hundred years or so, so you decide to wipe out humanity, in fact, literally blow up Earth.(a bit of overkill if you ask me, if a stealthy virus could have done the same job, but OK). That's no trivial task, so you have to design and test in several phases. That's all understandable. But why did they test one of their designs on earth itself? I mean, I can understand that you want to perform certain test in environments that are as much like the target environment as possible, but still ... did they hope humanity wouldn't notice or care? Did they hope humanity wouldn't be able to find out who did this to them in time? Why spoil their element of surprise in the first place, given how many inhabitable (but uninhabited) planets there seem to be they could have chosen for their test?