Was the Tree of Knowledge of Good and Evil bad from the start?
That tree, including its fruits, was as perfect as the rest of Paradise, even 'a delight to the eyes and desiring to make wise'. But Adam and Eve misapplied the fruits by eating them. Try eating a MacDonald's billboard and you'll get it. Also see Proverbs 3:5.
There is no mentioning of an apple in this account. The fruit of the tree of knowledge of good and evil may just as well have been a banana.
The Tree of Knowledge of Good and Evil is sometimes referred to as the knowledge-tree, as if the Bible would promote ignorance, which it obviously doesn't. But this forbidden fruit-thing is not a matter of cognition but a matter of ethics. It's the fruit of the knowledge-of-good-and-evil.
Before the fall, Adam and Eve had only knowledge of good. When they munched on the banana, they also gained knowledge of evil. Their knowledge of good was apparently not mature or strong enough to withstand the effects of the knowledge of evil. Evil entered them, and that killed them.