I take this to mean that with the training that Luke was given, he could possibly help his friends ("you could"). Yoda thinks that he won't, hence the "destroy" part ("you would"). Yoda thinks Luke is reckless, and "now matters are worse". But, since he could not foresee the exact outcome of events, he left in that possibility that Luke could help. And I guess he didn't realize the third possibility that actually happened, not helping but not joining Vader either... well perhaps he did, in a way: "mind what you have learned, save you it can".

Edit: Guess I need to watch the movie a few more times.. losing my grip on the exact dialogue