This is a very complicated issue. There have been many crimes committed by both sides. The FARC themselves came out with an apology on all the victims they have kidnapped over the years. I think the point of the prize is not to delete his older actions but rather to congratulate him for his attempts to finally bring an end to a civil war that has costed hundreds of thousands of lives over the years. Just like the rest of the prizes are given for specific discoveries by scientists, and not for their every discovery during their life's work, similarly his action on trying to end the civil war was deemed worthy. Additionally actions are judged relative to one another and 2016 has not a good year for peace actions.