But that's not your job. That's no one person's job to be judge, jury and executioner. There's a reason that's no one person's job (and it's partly to protect the person doing the job) and also, yes. I really don't think many people would argue that there's NOT a difference between killing a person in cold blood and putting down a rabid dog.
Reasons and context should matter. And should not be the burden of one single individual. There's a reason we have a distinction between the three branches of law. It's checks and balances.
And what? Subjective? I don't think it's so easy to find an actual, willing to go through with it significant group of people who'd prefer being dead to being not dead.