The end of Season 5 is meant to bring Oliver’s journey from the pilot full circle, which means that he really should end on a high note of sorts.
The murder of William is something that Arrow and Oliver might not be able to come back from, and the show is already dark enough as is.
All of this said, I’m not sure that Arrow should really go as big and bold as to let the hero’s child be murdered by a bad guy.
Maybe not quite as high as driving off into the sunset with his lady love as he did in the Season 3 finale, but definitely not mourning the death of a kid who died because his biological father made an enemy of a madman.