The definition of "Good" or "Bad" changes from person to person. Some people think killing people is Bad, but other people think burning witches is Good. And no one wants to do "Bad" things. Everyone wants to do "Good" things. Someone is "Bad" only when their definition of "Good" contradicts your own.
Hitler thought he was doing the world a favor by getting rid of the Jews. Stalin thought his five year plan would benefit his country.
Now, this doesn't justify these people's actions, but it does show that no one is inherently evil. Everyone is "Good", their actions just depend on their definition of "Good".
Yes, in the world there are many people who go out of their way to improve other people's lives, even sacrificing their own, and many people who care nothing for other people's well-being, often actively despising it. But people don't fall into neat categories. Often great selfless souls will work together with cruel heartless bastards against other people both good and bad, for all sorts of causes, for better and for worse. In all too much fiction, all the good people are working together for a just cause against all the bad people working for an unjust cause. Life doesn't work that way.
It all depends on a persons perspective and definition of good. Take two people fighting in an argument, one says blue is the best colour and the other says blue is a disgusting colour. Both aren't wrong, yet both have a following who agree with them.
In that case, it depends on those two to figure it out, but when applied to war per se, it's basically a copy and paste from the two fighting. Because both sides believe that their side is the "good" side. Only difference is, is who wins.
In our society while it is true that there are things which are morally wrong, that's just the thing, it's our society. There are some universal things generally considered moral to humans here on earth, such as, do to others as you would wish upon yourself. Yet, other species here on earth don't seem to follow these rules, some do, but some don't. Besides the point how can we know that we aren't living in an Orwellian society right now, as I type this. Is it such a bad thing then? I have a pretty happy life. I feel the same philosophy can be applied to what is good and bad. Back to a hypothetical species, "good" is simply to reproduce, to continue, to perpetuate oneself. So to them, killing other competition is great! That's a competitive society. In a cooperative society (us) it is bad to this because previous strategies were not formed by this philosophy. So as you can possibly see, good and bad are simply two sides of the same coin. To our human perspective there is such a thing as good and bad, but overall is there really such a distinction?
Space is limited but here are a few ideas.
I believe simplifying the world to the good guy - bad guy binary misses the whole gray area and presupposes that simple remedies such as getting rid of bad guys will solve the problem. Everyone has the potential to be "bad" or "good," it depends on the circumstances, their upbringing and, in some rare cases, on mental illness, how they are going to act in challenging situations. If someone takes revenge on someone for injustices brought onto them or their dearest, are they a bad guy or a good guy?
We should cherish the good sides and invest more in restorative justice, give those who, for various reasons, acted in a bad way and hurt others a chance to remedy the wrong or do good in order to make a meaningful contribution to the society. I know this sounds idealistic, but I am talking about principles. Eliminating people in order to punish them for their behavior may not be the best option we have simply because the hurt they caused remains. Changing their behavior and making them realize their mistake may make them want to improve and help others.
In war there are no bad or good people, just people who kill each other. One side will always see the other side as the "evil side"... So is there really a good side and a bad side?
Even when it's a question of morality, morals are a concept that everyone individually perceives, morals aren't written out. So if morals don't exist, what people do that go against your morals doesn't necessarily make them bad, just bad to you. Committing genocide is generally seen by about 90% of people as a terrible thing... Because 90% of people have a somewhat similar moral compass.
Nothing is good or bad, it's human nature to simplify things and perceive them as such.
If I have to break it down even further to an example using people in general, then here:
There are people who commit serious crimes, and even if you decide to see things with a moral filter on your glasses, you might not see them as just "A bad guy". People (including criminals) have a lifetime doing "bad" and "good" things. Most criminals probably have loved someone, done something for someone, tried helping a living organism, or even just do things they think are to help or that they think are good. And "good" people have done "bad" things too, if you're going to define someone for their actions you should include everything they've ever done. No one is good or bad, there's no such thing.
Everything in our universe is gray (symbolically) but for some strange reason it seems that all the good and bad happens on earth, well that's wrong. We only think that because it's all just human interaction, and it's all human psychology.
You have guys who honor and respect women, defend and protect them. Those are the good ones. And if on top of that they have good manners and are acheivers, they are great men. And then you have those who use and exploit women, repeatedly cheat on and abuse them and bash them. Those are the bad ones. "Relativism" is cute and dandy, but categories exist and let's just be honest about it. What about men running female trafficking for example? Good is good and bad is bad. Doesn't mean people can't change, but that hardly ever happens, and what they are at that moment is what they are. There ARE good guys and bad boys.
My partner = A good guy
Why? We've got 6 years under our belt at 21 and he still tells me he loves me every day and goes out of his way to make me happy. He supported me when my uncle (more like a father to me, raised me) got cancer and when my mother relapsed to heroine, then got clean, then relapsed again and continues to support me while I struggle with her addiction.
My uncle = A good guy
Why? He was left unable to have kids of his own after cancer as a teen, but he raised my sister and I with my Nan when my mother could not. He's never hit a woman and has never been even verbally abusive to me or my sister in our lives. He didn't even hit my when she totally deserved it (In a drug addled rage attacked me, about 6/7 years old then my nan, my uncle just restrained her). He's had cancer 6 times since he was 17 and hasn't been able to really live but he hasn't turned to drugs or become abusive from depression... Something common in these parts...
My sister's baby-daddy = A bad guy
Why? Left her after two months, leaving her 2 months pregnant. He was 17/18 and her first, she was 15. Then he brutally attacked some poor couple with his friends and spent the first year of his sons life in prison. Then once things calm down, he's out of prison, been seeing his son once every 2 weeks or so, my sister get's back with him, she's the happiest she's been in ages - He's her first love and the father of her child after all - Then he decides he doesn't want to be with her after all.
My Grandfather = A bad guy
Why? He molested and raped my mother and aunt.
My Great-Grandfather = A bad guy
Why? He molested and raped my Nan's sister and her friend.
However, in the world humans have crafted, there's a great line of morality that needs to be followed.
I do admit that many wars between opposing ideologies were created just because we all wanted to improve the world but simply had different methods, but does this means there are no "good" or "bad"? Of course not, and sociopaths and psychopaths are living proof. Some people are more charitable and selfless than others, Idealism and cynicism aside.