if he had angels guarding the tree before the fruit was taken sin would not have happened
God told Adam and Eve that they could eat of any tree in the garden except for the tree of the knowledge of good and evil.
That means Adam and Eve could eat from the tree of life, and it would be alright.
Which Adam and Eve probably ate from the tree of life, especially since it was the most important tree in the garden, and Adam and Eve knew what the tree represented, and being in a sinless, innocent state, it was alright to eat of the tree of life.
It would of never entered Adam and Eve's mind to eat of the tree of the knowledge of good and evil unless an outside source tempted them.
So God allowed Satan to tempt Eve, and she told him that they could not eat of the tree of the knowledge of good and evil, for that is all she knew, and could think.
But then Satan tempted Eve by telling her that if she ate of the tree she would have an elevated position, and be greater than her current position, for he tempted Eve with an alternate reality than what God gave her, and Eve sinned.
Then Eve was the outside source that tempted Adam by showing him she did not die, and Adam sinned.
When they sinned then they did not get to have it good in the garden of Eden, but had to fend for themselves outside of the garden, where all they had to do was dress the garden, but now they had to labor with harsher conditions.
God put them out of the garden, and then he protected the tree of life, lest they eat of it, and live forever, for if they would of eaten of it in their sinful state they would of stayed that way forever, and since Eve is the mother of all living God would not let that happen for then there is no salvation.
Also Adam and Eve knew what the tree represented being the tree of life, and they might of wanted to return to the garden, and eat of the tree of life and reverse the sin that they committed, and put things back to their original state.
When Adam and Eve were put forth from the garden it was not the tree of the knowledge of good and evil that was protected, but it was the tree of life that was protected.
Also God would not protect the tree of the knowledge of good and evil while Adam and Eve were in the garden for God gave them a choice, and that tree was the choice.
And also if God did protect the tree of the knowledge of good and evil while they were in the garden they would of never sinned, and Satan would of never tempted Eve, which God allowed Satan to tempt Eve, so He would of not protected the tree of the knowledge of good and evil while they were in the garden.
It does not make sense for the tree of the knowledge of good and evil to be protected while they were in the garden before they sinned, for then they would not have a choice, which is true love, and Satan would of never tempted Eve for that was the only tree that they could not eat, and God was allowing Satan to tempt Eve.
They could eat of the tree of life all the day long, until they sinned then God had to protect the tree of life.