ManTimeForgot
Temporally Challenged
Here's another formal proof:
1. Genocide is bad. (Premise is false since actions by themselves have no moral weight)
2. Therefore, a morality that condemns genocide is better than one that does not. (Conclusions which follows from false premises are invalid)
3. My morality condemns genocide; yours does not, if God permits it.
Therefore, my morality is better than yours.
Killing in a vacuum of circumstance is not wrong. It is very likely wrong, and that is why your argument is inductively quite strong. But killing by itself cannot be wrong because there are situations in which killing prevents more harm than it causes (does more good than harm). By logical extension of this there could conceivably be a time and a place for genocide.
If it somehow became necessary to exterminate a sub-section of the human population in order to prevent them from sterilizing the surface of the earth, then genocide would be moral thing to do. The plausibility of this particular scenario stretches credulity to be sure, but it is not impossible. Any moral individual would like to believe that there would always be a better way than large scale killing, but the truth is more more gritty and hard to swallow. Sometimes we do not know what that "better thing is" and waiting around until we do would allow more harm to be done than would be done if we act with a "sub-optimal" plan now.
Anyone morality system which rejects any action absolutely is immoral at its outset as it restricts available actions during the variety of circumstances that life possesses. Unless you happen to have flawless foreknowledge (psychically knowing the future or what not), you cannot possibly be aware of what life will throw at you, and as such you have to be able to keep your options open (at least in the theoretical).
In practice genocide should be condemned. We do not want people in general to believe that this is a generally useful option... We should strive to work out our problems with less harm done. But completely ignoring any option would mean that we would be unable to deal with situations in which even greater harm stands to be done. It is this logic that stands at the root of "We do not negotiate with terrorists." No matter what the terrorists threaten more harm would be done in the future by giving in to the demands because more demands would get made. There is, however, a breaking point. If terrorists are capable of threatening a whole society/country, then capitulation is very likely morally necessary. You would have to undertake "covert" methods to undermine such a threat, or be willing to "take on for the team" so to speak and let your society be destroyed so that other societies/nations do not suffer the same fate.
MTF