Q. Why was the tree of the knowledge of good and evil in the garden to start with? Was God just trying to trip them up?

A. No, The tree wasn’t a trick or even a test. The tree was there to protect the man and woman from an enemy who was present but could bring no harm to the couple except through disobedience in regards to the tree. It was a warning.

Q. How could this tree "protect" them? How did this tree "warn" them?

A. Protection comes through obedience to God—period.

The man and woman did not need to understand anything about evil in order to be protected from it.

What they needed to do was obey God.

The tree was there to warn them and protect them, because the devil was there to trick and trap them—not God. God, in his wisdom, arranged it so that the enemy had no access to the couple

...except through that one tree.

By placing the tree in the garden along with the “one commandment,” God successfully restricted the access and power the enemy could exercise over the couple. He reduced the number of points of contact to “one.”

The enemy's plan was to cause the couple to "disobey" God, because he knew the moment they did so, their innocent sinless existence would be gone forever (along with the divine protection that came with it).

He knew they would then become subject to him and all his oppression.

To have the knowledge of good and evil is a grievous thing, because the only way they could acquire it was through first hand experience—i.e., to sin; which they did the moment they reached forth their hands in direct disobedience to God's command.

The tree was not there to trick or trap them. Had the tree not been there, the couple would have been sitting ducks with no defenses whatsoever, because they would have had no knowledge that evil even existed ...and the serpent would have "still" been there carrying out his agenda. God provided them with a safe guard—they did not have to know they needed one. They only needed to obey the "one" commandment, and all would have been well.