Originally Posted by TrickyNicky
Not necessarily. Robert makes mention of killing "him" in his dreams over and over for what happened with Lyanna. It is a reference to Prince Rhaegar, whom Robert killed in the Battle on the Trident during his rebellion.
We're sort of treading on spoilery ground here, so should probably be careful. At this point in the story I think you're correct in that we (the readers and now viewers) believe it was Rhaegar that killed her. And I'm 99% sure Robert mentioned Rhaegar by name last night, when they were eating together and talking about the past.