Because its exactly the very problem you describe.
People do not need an excuse to kill.
They kill as a result of their morals/believes.
It is true they do not need a reason. I am sure they use an excuse to either convince someone they are good, recruit help in their pursuit or avoid a conscience that is influenced by society or may derive from self (Trying to cheat their conscience).
And if you think thats wrong, well so do they, just like you, but on the other side.
I think it is wrong and I do not want to kill because of it. I mean I might harbor resentment between a known killer. I would not want them dead because I do not want to pretend i know what life is like being them. I know I do not know their experiences and the way their mind thinks. I can assume that in their situation I might have even done worse.
The main reasons I think it is wrong...
1. World chaos.
2. I would live in fear. I would live in fear of being killed every day if it was no longer part of the law. I would rather not have to fight to survive every single day. It would be more apparent that there is always an us versus them. I like happiness and it is hard to be happy when I would be in constant fear. I think most killers would rather not killing be legal because they may want to be one of a few killers. I doubt they would want to have everyone be able to kill them at any moment.
I see not killing more as a mutual contract in fear of being killed. I know that if I were to clone myself I would not plot to kill my clone even though they creep me out. It is simply because if I am willing to kill them I know they are willing to kill me. I would rather not watch my back every second wondering if I will kill them or they will kill me first... I feel like this is why there is "treat others as you want to be treated". It is a bit vague. In the end it shows that if you treat someone right they are more inclined to treat you right. In the animal kingdom there are bats that will get blood and give it to a bat who did not find blood if they know that bat has helped them in the past. It is a bit of a survival tactic. I think it is wrong because I do not want it done to me and I am willing to not kill. I prefer the mutual agreements of helping. I can see some people would want to fend for themselves and maybe would not mind every man for himself. I hope this makes sense.
The only real side is your own, but when you realize that, you can't judge others condescendingly for doing the same thing.
Now who you think is on your side and etc, that is subjective and not relevant in the general theory.
There is always some level of us versus them unless the person is able to value someone over them self. It has varying levels of intensity. I think the only time it might be gone is if a mother dies for her child. I mean she put someone before herself considering that person almost part of their identity of self. It is show that when couples break up they feel they lose a part of them self. It might still have a small degree of disconnect. I know the child might still be a "them". It is subjective to the person how they label the levels.
If you think you do not live by them versus us, you are wrong, unless its a me versus them thing. Because there will always be someone who do not fit with your subjective ideas and that someone is automatically a "them".
I do have a "me" vs "them and distance between individuals. In certain case I might even kill. I know I would defend someone I love from death and kill to protect them because I consider them "us". I want to use this knowledge more to create less of distance and a barrier between me and people I want. I know I see people sometimes and I just want to have them. In high school the first time I saw a girl I told myself I want to be her friend and now she is my best friend. I want to use this to break distance and barriers.
I doubt I could break the barriers all over the world to create world peace. I mean if world piece were to be achieved there would have to be unity and less distance. It would mean considering everyone "us". It is shown in some movies when all of mankind units to fight aliens. There is only unity to fight off the one of greater distance. I know people actually trust their enemies slightly more than strangers. It could be because they feel they know their enemy and know what to expect from them. I tend to respect my enemies for their honesty and I do value them over a stranger.
There is nothing wrong in any act by itself, its important why its done.
I think intention is the most important.