Synonyms for Putting to sleep:

v.

•killing (verb)
slaying, Assassinating, executing, smothering, Exterminating, hanging, suffocating, Removing, chilling, doing away with, Murdering, Massacring, icing, Eliminating, Snuffing, Slaughtering, asphyxiating, Drowning, Sacrificing, putting down, strangling, purging, butchering, Smiting, doing in, Dispatching, Liquidating.