The Permian-Triassic extinction event didn't just kill of 90% of all life. It killed of 90% of all species - that is, it killed off 100% of 90% of species. Of the remaining 10%, it killed off 99% of some species, 98% of others, and so on. It was frighteningly close to sterilizing the planet.
Humans do have the capability to actually do that - sterilize the planet. It's highly unlikely, but possible if the entire world economy were dedicated to that - and it could be, as a side effect, because of two important effects:
- The result of all technological progress is to allow people to do things they couldn't before, either by making something new possible, or making something existing available to more people.
- There will always be some fraction of those people who are sceptical of the consequences, ignorant of them, or think they can get away with it just for themselves.
This means there will be a steadily growing number of people who are willing and able to do an increasing amount of damage in pursuit of their own goals, and if those goals result in hugely profitable corporations that can influence (or ignore) government policy throughout the world, extinction of all life could then become the main product of nearly all human activity. And humans are pretty good at accomplishing their goals.
To be fair, at some point the consequences will be obvious and the number of people willing to continue will fall. But that's as likely to be too late as not - see Rapa Nui (Easter Island) for what tends to happen then. And see Venus for how bad it could get.