@Sorou Classmate, Kamimura Yuuka wa Kou Itta. and it should fit the request

Sakura Sakura (Morishige) - not last people on earth, but sort of last people in Japan; starts of with an interesting Truman Show feel, but unfortunately dissolves into ecchi fest after a while.

World's End Harem - not last people on earth, but last men on the planet (there are still plenty women); this H setting has plenty potential for an interesting story, but unfortunately the manga only delivers the expected trashy fanservice... (If you want to check something with the same setting, but with actually decent plot, I recommend Polish movie Sexmission/Seksmisja; it's still a bit NSFW though)

Chikyuu no Houkago

("Earth's After School Rest")

Yokohama Kaidashi Kikou [url]http://www.mangaupdates.com/series.html?id=586&comment=104412]

(post Apocalypse. Is humanity dying out? The main character is a lovable android. No killing happens. It's unique!)

I was about to post Chikyuu no Houkago too. By the way cecropiamoth, if you write the title between m tags ([m]title[/m] without the _ characters), it will be linked automatically (manually copied links tend to work badly, as you get logged out if the http/https protocol doesn't match).