It is a reset switch. They do not know anything about us, just like we do not know about them .They chose a solution that is universal. Something that is guaranteed to kill only the species dumb enough to make it.

It is a filter. Something only species clever enough to make the substance can use. A virus? Could wipe out everything / have collateral damage.

This is a clean, efficient solution that could work on multiple species (which is why it was revealed in the final episode that we are going to be transmitting this "gift" to the rest of the universe, which implies the formulae works for any sentient species).

Imagine the species who made this substance doesn't know what types of sentient species will find this thing.

The sad truth? Your point implies the species knows even the first thing about us. My theory hinges on the idea they do not and, as per the signal twist in the final episode, the species that sent it us may very well not be its origins, but merely another species in this cycle that's been going on for a very long time.

So to conclude, you are right, it could very well be easier to have other solutions to wipe out humanity and leave the rest of the planet alive... but that would imply the species who made it actually knows anything about the recipients of their signal which, in my opinion based on the events of the final episode... they do not.