(2/2)
Obiwan realized he was wrong because he'd forgotten his Jedi training. Again, Sith are never the real enemy, rather it's the struggle against the Sith within. A struggle he lost just as Annakin did, the day they created the Darth Vader we saw in A New Hope.
But fortunately for both - and for Luke - it's a struggle that never ends. no matter how lost in the dark side a Sith is, or how strong a Jedi's mastery of integration and balance.