I've always viewed the day they broke into the van as truly being the point of no return. While it has been pointed out before, I do find it very interesting that Dylan did the RNN interview the same day. It's almost as if you are getting a glimpse of where his life could have gone had he and Eric not broken into the van that night.
I think Dylan was very good at hiding his true feelings, but I do think he was being somewhat genuine in the video. The interview did occur before the break in. For example, I don't think he was genuine about going to look at colleges with his family. He knew by then that he wasn't going to college, but he had to play it off like he was excited to go. I believe he and Eric also saw the van break in as being the point of no return. Once that was done, the massacre was going to happen come hell or high water.