Sorry it's a tick of mine, if he said "we're killing trees and animals" I wouldn't have said anything at all, but saying we're killing the planet, it's just a stupid expression.
It makes sense. We're slowly destroying the planet's ability to support life. Which, eventually, would make the world a lifeless planet.
Without life, the world would be considered dead.
We are not killing the planet.
We are killing the planet's ability to sustain Life As We Know It.
The notion that the Earth is something that will be killed by our destructive tendencies is a completely wrongheaded means of framing the environmentalist agenda. It presumes a moral responsibility.
But really, environmentalism is more than just a moral responsibility to Earth and our non-Homo sapien cohabitantsm, it's more than just an ethical duty to use resources judiciously.
Rather, environmentalism is about saving humanity. It's a pragmatic intellectual concern. In other words, to NOT preserve biodiversity and to NOT fight the causes of climate change is NOT immoral or unethical. It's stupid, it's the equivalent of human civilization putting a bullet to its head.
The myth of "saving the planet" conceals the fact that the Earth will be just fine, thank you, despite the crazy things industrialized human civilizations can do. Granted, vast numbers of species will perish and we will die from climate induced effects and starve from unsustainable agriculture, but the Earth will STILL be around.
Earth has survived giant asteroids, super-volcano eruptions, massive ice ages and other 'apocalyptic conditions' that the self-destruction of humanity will just be another passing fart on the face of the planet.
The planet is a durable thing that can recover from disaster, but human civilization is far more fragile, being entirely dependent on the Earth being in a very specific biological/climatological state of affairs, one that humanity is destroying. In effect, environmentalism is about humanity stopping it from killing itself.