I think how dangerous Knockturn Alley is can be subjective. It has an obvious Dark slant to every store and item you can find there, but I think Hagrid telling Harry in CoS that it's a dark and dodgy place probably had it set in Harry's mind and then the reader's mind that it was just that, dodgy. I'm relatively certain the fact of that was reinforced when it was revealed the Voldemort worked at Borgin and Burkes when he left Hogwarts.

To think of it rationally, it can't possibly be as dark as they make it out to sound in the books; sure there are dark stores, but like someone else already said, Hagrid was able to buy magical gardening supplies. Can't be all bad when you really think about it. I mean, they even have their own robe fitting place there, Twilfoot and Tattings.