We barely knew Atom (Rhys Ward) and to be honest, his death didn't make this list because we were heartbroken to see him go. Atom's death is devastating because of what it meant for Clarke. When Bellamy, Charlotte and Clarke stumbled upon his body, which had been ravaged by acid fog, Atom begged his friend Bellamy to kill him. However, with Bellamy unable to go through with it, it's Clarke that wound up having to fatally stab Atom in the neck while she did her best to make his death as comfortable as possible, even humming to him as she stuck in the knife.
Photo Credits: Steve Granitz, WireImage; photo: Prashant Gupta / FX Networks; CBS Photo Archive, CBS via Getty Images; J. Vespa/Getty Images; ABC; NBC, NBC via Getty Images; Getty Images/NBC; Randy Shropshire/Getty Images; Sonja Flemming, CBS