Not to butt in,(but I am) here are some thoughts:
But again, what is the purpose to evil?
To bring creation to ruin and separate man from God, eternally and irrevocably.
In the "Free Will" thread, you posted that since God knows all of our actions, thoughts and decisions before we even make them that indicates that free will doesn't exist or is an illusion. (If I am mistaken, please clarify your position for me. Thank you.)
OK. Simple. Because you know someone is going to do something, does that mean they have no responsibility for their own actions?
Justice has one dimension: Punitive.
Mercy has one dimension: Forgiveness.
Consider: If you create a rational and autonomous nature, that is a self-determining creature, say a human being, or an angel (and all angels are rational, like us, but not all enjoy the autonomy that we do) and they make a mistake, do you try and put them right, or do you simply blink them out of existence for their error.
Rational creatures are aware of justice and mercy: when the operate together, man learns by experience, and can grow. When they operate without reference to each other, he doesn't, and he learns either fear or contempt.
Why would God create a condition that would create distress, violence, terror and death when he already knows what the outcome will be?
You assume that Adam and Eve were obliged to fall. They weren't, each had a choice, and made it ... the wrong one ... knowingly and willingly. Do yo always do what you know to be wrong?
They did not know the outcome of their actions, but they didn't need to, all they needed to know was it was not the thing to do. You steal a car, it's a secondary issue what happens to the person you stole it from, what is salient is that you know stealing is wrong. If you know that a bad thing will happen to someone because of your action, then you are doubly at fault. That you didn't know what would happen does not make you innocent of the theft.
Same with Adam and Eve.
Then factor in the world is not God — it has its own nature, its own natural justice. Why would he create a condition that causes pain if you stick your hand in a fire, and why does the would heal if you pull it away in time? Why will you fall if you step off a cliff and not float up in the air? Why does water come out of a tap, and electricity out of a socket (OK, bad analogy, with my DIY skills, there's always the chance ... )
The world is the way it is, it is relative and depends on each thing living in relation to every other thing ... and God says, this tree is here as a symbol of your autonomy and self-determination, and it's fruit is a symbol of the attraction of bad things, things that will upset the peace of the world. Even though they look good from the outside, believe me, the harmony inside will vanish in an instant if you taste it ... so don't touch, OK?
Why did God place the Tree of Knowledge in the Garden of Eden?
You might well ask why did God give man free will if He didn't want man to make a wrong decision? Answer: Because if man didn't have free will, he would be an animal, not a human.
He must have known beforehand that Adam and Eve would taste the fruit.
There is no 'beforehand' in God, but that's not the point. The point is that complaint is trying to shift responsibility for the act off the self and on to God. You might as well complain, "why did God make me human and not a mouse?"
Consider, all the animals lived in the Garden, and more than one, a fly, a bird, a monkey, whatever, must have tried and tasted the fruit ... but that didn't bring about the Fall, so the fruit has a symbolic dimension relevant only to autonomous self-determining beings ... the fruit means the tendency to do what suits me, at the cost of my relationship with you ... that's a temptation that faces us every day, and we fall daily ... but it's still our choice, we choose to succumb, we are never, as much as we try and kid ourselves, overcome.
Regardless of eternity, the act is and always remains our free choice. You chose it. God didn't make you do it, and nor did the serpent, there's no work-around on that one.
Thomas