Deprivation of life, be it of someone or something, has always been a topic of must debate the world-over. Religions, organizations, political parties and whathave you all seem to have their own stance on the matter. You have those that clearly condemn it such as with certain practices of Buhddism and shall not even harm the smallest creature, to those that justify it as within select examples of Islam, and then you have the clearly hypocritical as portrayed by old-age Christiniaty when their 'crusades' amounted to little more than sponsored slaughter.
Wherever you go and whoever you are, you need to understand that there is someone somewhere else that vehemently disagrees with you. There is no global standard and people have always been trying to establish one, but as we can see throughout history and to the present day, none have managed to convince the remainder of the populace that they were right. It just won't happen so long as we maintain our free-will in the matter. You might think something on the matter of killing, but chances are there is some organization across the world, some local religious caste, or even your neighbour who subscribes to a different political party that thinks otherwise.
This is unlikely to change in the near future unless mass-communication becomes flawless and the global population becomes localized in one area, allowing for a cultural homogenization. Such an eventuality (at least the latter) is extremely doubtful in itself and there would probably be people to oppose that issue as well.
It depends on whatever you believe then? Is that it? Seems a bit of a cop-out. Yes, it is, because whatever might be said in this forum amounts to very little in the end, on a global or even personal scale. Though if you are still interested in purusing the concept then I have my own reasoned beliefs noted down underneath .. For as much good as they will do!
In western cultures a common belief (as seen in the law that governs) is that self-defence is justified means of killing. After all, isn't it safe to assume we'd all rather not die ourselves if we have the option of preventing it? Still, though, you're not expected to go bragging about it. Why? Simply because, killing is still tabboo and sometimes prosecutions still go ahead if more than adequate force was utilised or certain other aggravating factors were present.
I don't believe the law is exactly moral in itself though. If you don't understand this it might be helpful to consider the alignment axis used in some roleplaying games, wherein it is split between good/evil and law/chaos. Neither law nor chaos are specifically good or evil. I have more backing than obscure RPG examples though.
At least over here in Britain, the majority of law students will tell you that the law strives to be equitable rather than morally just. It doesn't matter if you feel sorry for the smaller guy in this business agreement with a substantially more powerful corporation, if he made a bad call and the terms of the agreement are solid then there's no justification to help the underdog. If the sole-provider of this old lady dies at sea whilst on a voyage for a lump-sum payment to be given at the end of the trip, she still doesn't get a dime because his contract wasn't fulfilled. Tough, but fair - perhaps morally unsound but equitable to both parties.
Back to the subject at hand though. Is killing ever okay? Examining the issue further, I don't think killing is okay even in terms of self-defence. It is still tabboo if you end up boasting about it, people sometimes end up terribly scarred by the experience, and in the end it just seems like people try to sweep it under the rug with a self-defence ruling and try to pretend it didn't happen. As previously outlined I don't think the law in general doesn't make many efforts to be a morally sound system. Completely accidental killing, wherein you could not have taken any measures in your power to stop it happening, would be acceptable on the other hand - but these are rare and far between - if you had no control over the situation whatsoever (such as an -unprecedented- mechanical fault for example that had no telltale signs of occuring before the incident) then it can't reasonably be considered at your expense.
Likewise, I don't understand the belief that killing some in favour for more or others can be morally justified. By numbers you are 'winning', but, would you really want someone to consider you little more than a numerical representation of a person? They are a person, they may not be saintly or even good or even deserve your kindness, but for all you know you might've been born in the same shoes and ended up in their spot - and they deserve life just as much as you do in that reasoning. Courtesy to them is to yourself. I don't understand the 'survival as a species' bit either in committing massacres. If we're willing to make such sacrifices of our own flesh and blood, so brutally and in a calculating fashion, do we really deserve to survive at all? Evil begets evil. You never achieve a morally positive act through use of evil means no matter the motivation. That would be like saying 1 + 1 = 3.
I don't even want to consider those situations where you supposedly have 'no choice' and 'must' decide. What sort of deranged madness is that? There are -always- choices, even if that choice is to not partake at all and let everyone die. You can either be part of the problem or part of the solution. There is no middleground in this.
By taking away life you are removing someone's right to exist. How do you make a judgement call on that, even? By their actions? Can you really understand their motivations? Think about all the times you've yourself been misunderstood, or someone has presumed something in error, or anything along those lines. How about all those secret thoughts and ideas that you have which will never know the light of day to another living person? I don't know, let's throw in another difficult factor, maybe they're mentally ill? For all you know this person is far more justified in their decisions and actions than you are in deciding to wipe them off the earth.
In the end though, you will believe whatever you wish to believe - and for that humanity is probably better for it.
Regards,
MultiMasky