You ask, "Why did knowledge of good and evil result in death?" The simple answer: Because God said it would.
This is not to belittle your question. It is a good question. We need not, however, make the answer much more complicated than it needs to be. When God said "you must not . . . or you will die," he was--in modern parlance--telling it like it is. Sure enough, they did eat, and they did die, both spiritually and physically.
Perhaps related to your question, if only tangentially, is the issue of theodicy, which is an explanation for the existence of evil in the universe. Ultimately, I believe, there is no complete answer to why and how God allowed sin to enter into salvation history, going as far back as Satan's urge to be autonomous and his subsequent banishment from heaven.
An answer, but only a partial one, involves the will, or volitional ability, that all sentient creatures possess. Frankly, there is a mystery to ungodliness, and we will never have, I predict, a totally and intellectually satisfying answer to why sentient creatures have the "freedom" to disobey God.
Moreover, there are many other partial answers to that particular why. I've likened all the partial answers to theodicy to slices of pie in a pie chart. No matter how many slices the mind can generate (and each slice is an explanation unto itself), there will always be a slice with a big question mark on it. There are questions, paradoxes, and conundrums that Christians will never have complete answers to. God does, to be sure, but we do not.
The book of Job makes this very clear. If anyone had a right to ask God about why he was being put through the wringer, Job would be the man. In the concluding chapters of the book, however, God does not give Job a complete and completely satisfying answer. In the simplest terms, God tells job that he is God and Job is not. God's grasp of things, his way of thinking and doing, transcend human understanding.
The Bible speaks of the "deep things" of God, things that his finite image-bearers cannot plumb fully. From Isaiah Chapter 55 come these words from the mouth of God:
“For my thoughts are not your thoughts,
neither are your ways my ways,” declares the Lord. “As the heavens are higher than the earth,
so are my ways higher than your ways
and my thoughts than your thoughts.
In conclusion, the Bible speaks of the "mystery of iniquity" (2 Thessalonians 2:7 KJV, or "the secret power of lawlessness" in the NIV), which has been in evidence for millennia but which will reach its denouement in the awful Day of the Lord. Perhaps then, true believers will understand more than they do now about God's ways and means of dealing with sin, sin that has been a blot on humankind since our first parents disobeyed God, and whom we all subsequently followed in their disobedience.