But wiping out all life on Earth would, while causing a temporary spike in suffering and misery, cut off the possibility of all future suffering and misery. I remember reading that at some point in time, humanity went through a "bottleneck" where the species was reduced to something like 5000 members. No cite, it's early, but take it as a hypothetical. If you'd been able to wipe out those last 5000, you'd have prevented the Crusades, WWI, the Holocaust, and the Cambodian killing fields, just to name a few.
By killing large amounts of people, you cause a short time spike of suffering and misery. Afterwards the people have wider access to resources, less cramped living conditions, as well as there being a lower amount of net misery available. And as you said "I don't care." Personal opinion, not objective.
That's not necessarily objective, though. Let's broaden this scenario: a human government based on Earth has waged a brutal, genocidal war against an alien species, and so the aliens decide to drop an asteroid on Earth to eliminate said government. It will end the war with the least possible loss of life, and though it will render Earth uninhabitable, it won't effect other human groups not affiliated with the Earth government (since they're on other celestial bodies). One could legitimately argue that that would make the world (that is the universe, our plane of existence) better. Edit: And of course, this still doesn't change the fact that your standard is subjective, not objective.
Oh, I like this question. What makes a hero? Well, that's very simple- perspective. Best example- Alexander the Great. To the Western world he is literally the great hellenizer and conquerer. To modern Iranians he is in their folklore as a demon, a horned creature from hell that caused death and suffering on a wide scale. Second example. The Avengers from Civil War. To some of the world they are heroes. To others, namely perhaps a family member of one who has died in a building explosion caused by the Avengers, they are a menace. Third example. A president could champion a bill that enables one young man the chance to go to college for free and achieve his dreams of being an engineer. For another, it could mean an additional tax that drives him from his home (one expense too many), and puts him on the streets. Perhaps later he becomes addicted to heroin, and dies of an OD. This is not a simple idea, this hero thing, it really isn't. Perspective and context are key.
A hero is somebody who does what's right no matter the cost to themselves. Somebody who puts others ahead of person interest.
First, your standard still isn't objective. Second, there are some grave flaws in that logic. For instance, let's say that someone--we'll call this person Morgan--is walking down the street one night when they see a group of people attacking someone. Our passerby doesn't immediately now it, but this is the early stages of an attempted gang rape. Being no slouch in some form or another of combat, Morgan intervenes to save the victim, and in doing so kills or seriously wounds all of the attackers. By most people's standards, Morgan is a hero. But by your standards, because they caused more harm than they prevented/relieved, they are not. We can scale this up. Let's say there are ten thousand members of a pacifistic ethnic/religious group. Their much more powerful and numerous neighbors hate them, for one reason or another, and decide to wipe them out. An international military force is dispatched to stop the genocide. Over the course of the campaign, far more than ten thousand people are killed or wounded. Does that somehow mean that the people who risked their lives protecting innocents from such evil are not heroes, just because allowing the genocide to unfold would have caused less total suffering and misery?
Someone who overcomes adversity and his own shortcomings while putting the needs of others before his own. And please note that this has nothing to do with loving a country, a corporation, a town, a city, a club, or any other non-human entity. Even on the battlefield, no one thinks: I'm defending my country. They think about defending the friends there on the battlefield beside them, the guys they go to chow with and share jokes with when there's a lull in the war.
