OK, firstly I agree completely with your description.
Secondly I have not pursued it very far for the same reason, it seems to take a lot of explaining to simplify it and also just the fact that they are such personal concepts that it is hard to generalise.
I will try.

There are the concepts of good and bad.
My apple tree has a great crop this year, that is good, the birds come and eat the apples, that is bad for me but good for the birds. We are in drought, that is bad, rains come, and flood the area, overall that is good for the farmers, bad for me if my car gets flooded. I have a lady over for the weekend, she wants to snuggle, I want to spend the day on the beach, sunny is good for me, wind and rain good for her. I have probably over done the simple bits so moving on.
I really love life, not just mine, I really love that things are alive. So anything that encourages life, as far as I am concerned is good. And anything that threatens life is bad. A nuclear holocaust would be bad. A volcanic eruption wipes out a forest but clears the way for new life?
Eventually the earth and solar system will die, good or bad? Against this back drop is anything else worth considering as good or bad because it will all die anyway? A nuclear missile is heading my way, there is no escape and I will be vaporised in 20 minutes. I go next door and shoot my neighbour because I really can't stand him. I get the satisfaction of shooting him, 5 minutes before he would have died from the bomb. Good or bad?

But what about evil?
For me only people can be evil. Evil requires ill intent. Hitler and the Nazis? IsIs? I consider them evil. The Catholic Church and the inquisition? Again to me evil. My own country's Government that is intent on continuing the extraction and use of fossil fuels at the expense of the environment for profit? I consider this evil. The fact that it will all disappear one day regardless, does that mitigate the evil? In 10 billion years, what difference will any of this have made?

So it all boils down to my own personal perspective, exactly as you have stated. My perspective, based on my values, limited to my existence. The concept of Isis as evil will be shared by possibly billions of people, but that is still billions of individual perceptions as opposed to any universal unquestionable concept of evil.

So what is the opposite of evil? Bad is the opposite of good, we talk of good and evil.
For me the idea of the opposite of evil could be divine as it fits with my old pantheist ideals that the Earth is sacred and the universe divine.
Or perhaps as I love life so much, then, LIVE is the opposite of EVIL. at least in spelling.