It was not stated as the killing curse in specific, but as murder. It was in a pensive scene with Tom Riddle and Professor Slughorn. It is towards the end of Harry Potter and the Half-Blood Prince.
Chapter 23: Horcruxes
"By an act of evil - the supreme act of evil. By committing murder. Killing rips the soul apart." -Slughorn
The phrase mercy killing is basically when someone is killed because they are dying from a painful disease. This process is usually accomplished by administering a large amount of pain killers.
a violent crime _________________ Killing Someone
abuse is treated like harming and killing like whipping someone and harming someone even like killing someone
by killing them
why did you kill someone
There's one for killing someone with a traffic cone, or killing someone while they're flying to their death.
Such dreams usually express anger or disappointment with the target of the "killing."
someone is in the act of killing someone else
Its basically killing someone.
If you can prove that you were in fact preventing them from killing themselves and not assaulting them, then under the Good Samaritan Law you can not be sued for preventing someone from killing themselves.
Someone killing themselves.
Plain and simple, you don't.