Lets hope some day you do not need to make that choice, but for now it is obvious you have never had to make that choice.
You will some day, be it your mother or father on their death bed, unable to speak, move, emote, you will need to decide to pull the plug, or leave them there a vegetable.
Then, are you a bloodthirsty murderer?
A mercy killing is just that, mercy. To end someones life who is already over, but to do so in a merciful manor.
Most of the time yes humans require consent, but sometimes it is for their better that you do not. Because humans very rarely know what is best for them.
There is a reason why in most tales humans are common to plea for help when a wound is obviously fatal, and helping them will only prolong their death and suffering.
There was 100% certainty they would all become zombies, we know that, he knew that, they all knew that, there is not a single person who was there, that knew what was going to happen, that did not know they were all doomed.
Just none of them wanted to be the one to pick up the rock.