I had two games where it happened in different ways and they are both memorable to me.
I have a multiple companion mod so in one game, it was Boone, Arcade, Ed-E and me going up to assassinate Benny. I wasn't really planning to do that though. I thought I'd just be able to talk to him and didn't know where it was going to go from there. Well, I guess Benny didn't like the way the talk was going because he pulled his gun on me. That's when Arcade turned him into a green pile of goo. Arcade was faster on the draw.

In the other game, Benny took off from the Presidential Suite after we were done talking and his bodyguards came in to take care of me while he ran away. Caesar captured him and he was bound and on his knees in Caesar's tent. Boone took care of Caesar so he and the Praetorians were gone. I went into the tent to talk to Benny again. and that's when I untied him, not really planning what might happen next. That game I had Animal Friend both levels. Benny ran out of the tent with the Legion dogs nipping at his heels and growling. I ran after them and watched as the Legion dogs took Benny down for good. Karma rules.