There is only one G-d, and creator of all. He gave one specific instruction in that lovely garden, they had everything they needed, food, companionship, and the most beautiful place to live anyone's ever seen, and the most important thing, love, a direct relationship with G-d.
They really had no need or want of anything, yet they were tempted to have more. Doesn't that still happen to us today? G-d gives us shelter, clothes, food, family, and yet we always want more.
They wanted to be like G-d, having all his knowledge, but with knowledge goes responsibility. He wanted them to come to him for whatever they needed, yet they wanted to be independent from him, and thus pushed him away. How many times do we do that today even?
Yes, the punishment was death, at that time it was spiritual and later on it was physical. The punishment was death because that is what they choose.
He said, from the tree of knowledge of good and evil you shall not eat.
But many, just like Adam and Eve, ignore what else he said.
9 The Lord God caused to grow out of the ground every tree pleasing in appearance and good for food, including the tree of life in the midst of the garden, as well as the tree of the knowledge of good and evil.
The one tree brought Life the other , the L-RD said, would bring death. They choose the one that would bring death.
They choose.
Today he still asks us to Choose LIFE! ( Deut 30:19)