There were both novels and comics about the city of slaughter. Murderous City was a science fiction novel by Liu Cixin. It told the story of Earth being invaded by aliens in the future. The protagonist was a reporter named Chen Motong. The comic was an American comic produced by HQ, and it told the story of a city of slaughter, created by Carl Brangail. These two works are very popular. I suggest reading or watching them.
Doujin moe net hentai mangas typically focus on explicit themes and might have unconventional character designs. They might also draw inspiration from popular franchises and give them a risqué twist.
Echii mangas usually have more adult-oriented content like sexual themes and imagery compared to regular mangas which may focus on a wider range of genres and themes.
Yes, mangas can be considered graphic novels. They share many characteristics such as being a combination of text and illustrations to tell a story. Manga typically has its own unique art styles and narrative structures, but like graphic novels, it uses sequential art to convey a plot, develop characters, and create a fictional world.