Farlander said:
Wanting to have the power to do what you want for your own desires (in the Jedi philosophy there's nothing wrong with death) is not a "good cause."
It's certainly a "good cause" if one puts it as "the power to save the ones you love". The fact that it transformed to a desire to do ANYTHING you want is another matter.
I remember when I was 7, that is even before knowing about Star Wars, I (after one rather unpleasant moment in my life) was sitting in my room thinking about a lot of things. One of them was - what makes me a bad person? I had a lot of questions, some of them on the if killing will make me a bad person topic. What if I'll kill to protect my family? I know the one I killed could have had a family too, you know. And so on and so on... Yeah, in my childhood I thought about things that I don't think any sane kid would.
If someone is attacking your family and you kill that person to defend them ("A Jedi uses the Force for knowledge and defense...") is a very different situation than committing wanton murder so that you have the power over life and death. Rephrasing it doesn't change the underlying difference.
Luke loved his sister, but he didn't let LOVE lead to so much FEAR for losing her that he was filled with ANGER that lead to HATRED that lead to him murdering an unarmed and defenseless Vader in Palpy's throne room.
Yoda and Ben both encouraged Luke not to go to Cloud City because he wasn't ready, apparently either with enough skill or wisdom. But Luke left to save his friends (too afraid to lose them), not murder Vader.
In ROTJ when he goes off Yoda says his last trial is to FACE Vader. Not kill Vader, but FACE him. But Luke is a wiser Jedi than Ben, because he sees that Vader can be redeemed.
The only problem is, all that doesn't make Luke less a tool. The reason Yoda tried to encourage Luke NOT to rush off is because he wasn't ready to, well, kill Vader.
And, about lying, there are many ways to lie to a person. Obi-Wan could lie about Luke's father a lot of ways. But he specifically said that it is Darth Vader who killed Luke's father, placing the desire for revenge (not the Jediest way to do at all) which would actually help their goal - kill Vader.
Yoda's training of Luke, with it's Cave-test (where Luke fails by killing Vader) and constant talk about how violence isn't the answer ("Wars not make one great") ({Jedi use the Force} never for attack") doesn't seem to me like a training regime for a tool of vengeance. Counterproductive even to what you see as Yoda's goal, which is encourage Luke to murder Vader.
Jedi battle evil. Vader does evil. If a violent confrontation with Vader is a possible, even likely, result of Luke becoming a Jedi and fighting to restore peace and freedom to the galaxy (....) that still doesn't make him a "tool to kill Vader."
As ghosts at the end Ben and Yoda both seem pleased as peach that Luke managed to redeem Anakin. Not really in line if they'd spent the last two decades plotting his murder.