In the comics, it's a complex situation. Different storylines might have different outcomes, but as of the common ones, no one has directly killed Homelander.
The death of Homelander in the comics was the result of a carefully orchestrated plan by a coalition of his adversaries. Each played a crucial role in bringing him down.
I'm not entirely sure as I haven't read the 'homelander graphic novel' specifically, but typically in such novels, there would be a protagonist who is likely to be the homelander himself. There might also be supporting characters like his allies or those who oppose him.