Depopulating a city is relatively easy and achievable with conventional bombs. A small country would only be moderately more difficult with nuclear weapons. After this, things get much harder.
Sure, enough nukes could finish off the human race, but that's out of the price range for most shoppers in this market. In the long term, climate change might do it, but who's got that sort of time? You're probably left with something like a biological agent, a pathogen of some sort, and still run the risk of pockets of people hiding away or being immune. Still, let's say you did it, it worked, humans are gone. Yay! You destroyed the world! Right?
No. You extinguished one species. You have a long way to go on the entire world.
So you put your mind to it. Defoliation agents for the plants, the cast of Deadliest Catch to get the crabs (you planned ahead and saved them from the nuclear fallout), a few billion tons of boric acid for the roaches-- get it all covered. Yay, party time?
Again, no. If you were meticulously thorough, you killed all life, but really the project has just begun. Everything is dead, but it's all still here. The Earth is still here. And if you thought rooting out the last extremophiles was hard work, you've never had to try & sweep 6 * 10^24 kg of mass under the rug.
Do you break it up? Is that enough? Technically it's still there and if you leave it it will all just accrete back together. So maybe you decide to push it into the Sun, either piece by piece or all at once, depending on on your level of godhood.
Finally, you can relax. Cut yourself a nice piece of "I destroyed the world" cake. Have a beer... ... ... Ehh, maybe don't eat the cake or drink the beer. You didn't really destroy the world, of it's matter sti exists. It's not gone, it's just molten.
So no cake for you. In fact, it will be a long long time before you see any cake coming your way.