@maxdancona,
I like utilitarianism which basically states that moral actions should be aimed at
maximizing happiness and reducing suffering.
"In utilitarianism, the moral worth of an action is determined only by its resulting outcome, although there is debate over how much consideration should be given to actual consequences, foreseen consequences and intended consequences"
Every situation is unique and there is no simple straight forward answer.
I think taking ideas such as utilitarianism into account when deciding whether something is morally justifiable is a good start. Genocide is obviously wrong, like when tens of millions of Russians (Lenin, Stalin) and Chinese (Mao) were systematically starved to death. Obviously suffering was maximized and happiness reduced. Everyday situations are more complex, and require deeper analysis.
Is it wrong or right to kill someone? Depends. If you could have killed Mao and saved around 80 million lives, suffering would have been reduced and happiness increased. What if you kill someone while defending yourself. I think that would normally be morally justifiable. What if someone robs you and you shoot them in the back of the head while they are running away from you. That's more difficult to decide. That's what courts are for. I think if you are flexible with utilitarianism, it can be a pretty good guide in deciding wrong from right. Of course there are other situations where that may not apply as well. Say someone's a career criminal and rapist, you are unprovoked yet you walk up and kill them. Well you may be reducing future victims' suffering in the long run, but you can't just walk up and kill someone. Is vigilantism morally right? That could lead to anarchy which may increase suffering...
...Anyways, I think morality is very complex and each situation is unique and deserves its own critical analysis.