I don't know how to predict Jack's eventually being either good or evil, because I'm not sure the writers are 100% sure about which way he will fall. I suspect they decided to see how Jack was received by fans with a plan A for if he was liked and a Plan B for if he wasn't. So far, most fans seem to really like him, so Plan A will come into play. If I have to guess, Jack will do some bad things, but his love of Sam will pull him back from the brink. This will be especially true if, as I suspect, Cas may not be our Cas. This is not a spoiler, just a hope because Empty Cas was a LOT more interesting than real Cas has been in forever.