By eating that fruit they proved themselves free will beings. Free will necessarily includes the capacity to do evil as well as good. Note that the fruit of the tree conferred the knowledge of good and evil, so they were no longer innocent children but adults, all grown up, capable not only of good and evil actions but of taking such actions in the full knowledge of the difference.
Free will is a great power and conferred a huge survival advantage to our ancestors.
With great power comes great responsibility. So that's where we are. I'm not Catholic but my spidey sense tells me they are on to something with the notion of original sin. We were as incapable of sin as any animal until we acquired the knowledge of the good and evil along with the ability to choose. So yes, sin is the universal human condition. That's not a bad thing: it just is.