Zanzibar is right about the bacteria. Saying that they are not an important part of the ecosystem is highly short-sighted. In fact I'd have to say they are possibly the single most important living part of the ecosystem. Without bacteria, life couldn't exist. Humans even rely on symbiotic relationships with several types of bacteria, and if they suddenly died in us, we couldn't live without them. They break down decaying matter back into soil and so many other things we couldn't even imagine. If they were gone life would probably cease to exist, at least as we know it.
Also, the euthanasia of people and bacteria naturally picking off the week are NOT good comparisons. Perhaps I misread what you wrote, but there are not many similarities between natural selection by natural bacteria, and choosing to die to avoid suffering. The key is choice.
As for extinction, I think there are tons of sides to this, like a complex dodecahedron with each side in various shades of gray. The only way we could totally take away our impact on the environment would be to all go live in the wilderness again and abandon all technology. No more farming, no more anything except our ancient hunter/gatherer ways. Even then our large population would have a large impact, though all told, we need agriculture and other technology just to support such a huge population on Earth. On the other hand, continual habitat destruction will have consequences that we probably know little of. What happens when many critical portions of the natural food chains go extinct? Will other organisms fill in the blanks or will the food chain collapse? What is known is that we cannot keep exploding the population of the planet without dire consequences in the future. While when it will reach a critical level is unknown, I don't think that anyone can deny it will without a significant change in the way we consume resources. The larger our population we will invariable be forcing out many many species, many of which will go extinct.
We end up caught in a catch 22, with both ends of the spectrum looking grim. We need to find a balance, try to reduce the footprint of our consumption, waste less, and work to protect what we have left. We can't completely stop our influence on the environment (which we ARE a part of don't forget), but we can lessen our impact. In the end if we want our growth to continue, I'm hoping we can colonize other planets and moons. Perhaps some day in the future Earth could be allowed to fall back to a more natural state as we spread out. I'm not holding my breath, but its the only hope I see.
