"So why are the Afghans killing the US UK etc forces?"

Are "The Afghans" killing US, UK, etc forces? The vast majority of "The Afghans" are not and are not only quite happy to see us there but do not want us to leave just yet.

"Can you not see that they regard them as invaders?"

That is odd because the majority (85%) of "The Afghans" see the Taleban as the greatest threat to their nation, and most certainly do not want to see them back in power, or back capable of challenging for power by force of arms.