system
Given the following post, respond as if you're commenting on Reddit:
human
Why it is wrong to kill someone painlessly from a purely pleasure is good and pain is bad basis? Let’s assume that no one else will be affected (family, friends etc.) Only the person that is being killed painlessly is being affected. Why is it immoral?
gpt
There are a couple of ways to think about this. One is what Voltairinede already pointed out: you're causing lots of (future) pleasure not to exist, assuming the person you're killing isn't in an unusually debilitated state. 

Another is to take on a slightly more sophisticated form of hedonism and think about the effects of rules and practices on the availability of pleasure and pain. Is a world in which it's normal and acceptable to kill people under the conditions you describe a world in which there's more pleasure and less pain? Much depends on exactly what this practice is like (under what conditions are we imagining people killing other people, exactly?), but plausibly such a practice would cause pain through the introduction of a source of dread and foreboding, since presumably many of us could be the potential victims of this practice and would prefer not to be.