At the most fundamental level, killing anything is bad because it deprives the killed from their opportunity to exist.. . At the human to human level, because humans have so much capability, killing another human eliminates the possibility of that human interacting with the world again. Each human has great potential and we - as a species - have optimism and hope that each human will interact well with the world and the other humans in a society. . . It is that hope that makes killing other humans bad. We hope that the other human could be a good person - a productive, loving member of our society. The fear of preventing that human from reaching their potential results in our feeling bad when that human is killed.. . The reality, unfortunately, is that not all humans consider other humans worthy of respect and co-existance. When that happens, we - as a society - have to band together and do what is necessary. In most cases, that means remove the freedom of the offending human to interact in society (e.g. prison). But sometimes split-second decisions have to be made to save another life (e.g. hostage situation) or a repeat offender will never learn through incarceration that hurting others is bad, so they have to be killed (e.g. capital punishment).. . In the global setting, some leaders believe that they lead through force. Their followers are willing (or enticed) into killing others in order to extend their idea of society onto other societies by force. The result is either genocide (if the victims don't resist) or war (if the victims resist and become opponents). . . Capital punisment is bad, but unchecked murder is worse.. War is bad, but genocide is worse.