There's a bit of a grimace at that, and Obi-Wan motioned for Luke to follow him. Being out in the open like this wasn't the best for this sort of conversation.

"Luke, when your father turned to the dark side, he was a great threat. Not only a danger to himself, but to everyone in the galaxy. I faced him, there was a duel, and I did what had to."

His words slowed and took on a sadder tone at the last sentence. Ever wonder why Darth Vader had that suit, Luke? This guy, right here.