A.) As a GM it's fine to make a certain option attractive. You can make it attractive to stab the poor old man, like describe how he hurls racist slurs if your psychotic character is also a minority, or describe his big fat purse if your psychotic character is largely motivated by money. Telling people what to do, on the other hand, is asking them to write your story for you, which is silly and a big turn-off for a lot of people. 

B.) It's always good to have established goals. Both short and long term. Whether you want everything to be known to everyone or not depends on the nature of the roleplay. Both sides have pro's and cons, which are to be weighed according to the playerbase and direction of a game. If you want a mystery-game and surprise your players, by all means hide them. If you want a game more about combat or romance than information-brokering, keep it out in the open. 

C.) Having a prophecy is fine, but blindly obeying it is stupid. Maybe the prophecy says something about a grand clash of blades at the last day of summer, but if I have a character who just wants the giant dead and at some point has a clear shot at said giant... They're probably going to take it. You need to be able to deal with this as a GM. Saying "Your character can't do this because plot." Is kinda like having invisible walls in video-games. It may be less epic, or less dramatic, but you can work around it and still have that clash. Maybe using bow and arrow is seen as cowardly by the enemy, a previous henchman takes up the mantle of commander and despite the loss of the giant, the evil army presses on because... They're simply not impressed or defeated; just enraged.