Obi-Wan knew that his best-friend and student, Anakin had fallen to the dark side. Knowing the truth, he was shocked in terror. Obi wan even told Master Yoda that he would not kill Anakin, because he was like a brother to him. But when Yoda convinced Obi-Wan that the Sith must be destroyed, Obi-Wan had doubts in his mind. After that, he went to Padme to ask about Anakin's whereabouts. He delivered the news that her husband had fallen to the dark side and the news regarding Palpatine as a Sith Lord. And when Padme was shocked about all of this, Obi wan needed to find him.
PADME AMIDALA: you are going to kill him, aren't you?
OBI-WAN KENOBI: he has become a very great threat (...) Anakin is the father isn't he? I'm so sorry.
So what was Obi-Wan's plan in going to Mustafar, Was it to kill Anakin or to redeem him?