If the aliens want to destroy all the humans on earth will they destroy everything in a second (booom !!!) like we won't realize we are dead and what we died of ..
OR they will attack us slowly i.e attack specific places and conquer them and then expand like in most of the movies.
They may be destroying all of us ,for bringing there part of their population on earth.
TL;DR Earth is big, and there are a ton of things living on it. It takes a couple tries to kill them all.
I'm assuming these aliens aren't so advanced that they could turn the whole world into plasma, summon a black hole, etc - more conventional, pseudo-"realistic" means of destruction will be addressed here.
Regardless of what you use - be it an antimatter death ray, gray goo cloud, toxic chemical element, electric charge, or some other thing that put a StackExchange user on the NSA watchlist -
Life finds a way.
Bacteria
There may be about 5 x 10$^{30}$ bacteria on Earth. They are diverse enough that nearly any concoction released into the biosphere will not kill them all. Even if you handwave a miraculous, kill-all serum, they've gotten to some pretty hard to reach places, so the logistics of killing all the bacteria on Earth remain a nightmare.
Tardigrades
Described as "the most resilient animal", tardigrades will survive most of what you hurl at Earth. As long as there is something to cling to, these guys will be around.
"They can withstand temperature ranges from 1 K (−458 °F**; −272 °C) (close to absolute zero) to about 420 K (300 °F; 150 °C), pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space. They can go without food or water for more than 30 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce"
From Wikipedia
Other Extremophiles
Some microorganisms are even better adapted than the tardigrades to survive deadly conditions. Read the above link to get a good idea of what these aliens are up against.
I would say more or less instantly, because if they are capable of coming here in a timely manner, they are just as capable at throwing relativistic missiles at us. They have no need to come here in person or fight any sort of war.
Here's a description of a relativistic attack: http://www.projectrho.com/public_html/rocket/spacegunexotic.php#id--Relativistic_Weapons--The_Killing_Star
If they are willing to invest enough energy, they can easily enough wipe us out from any planetary surface. I imagine space habitats, which could potentially maneuver, could be more difficult to hit, and if they expect us to have a self-sustaining population in space I suppose there might be a mop-up fleet coming afterwards. But if the attack came today, there would be no one left to meet it.
First of all: Does aliens want to kill humans and only humans? For example to free the world from the demented child humanity have become?
If yes they would need to engineer a fast duplicating virus/bacteria/toxin that could affect only human DNA and could easily be transported through air and water.
Or do they would like to kill mostly everything? Then they would need to just change the planet atmosphere into something that would kill things that need air to live. I.E. add more C to the O.
