A story like this one was well-nigh inevitable.
The Merlin Wiki has a full summary here.
As a younger man, Arthur was, essentially, the general of Uther's army. It makes sense, therefore, that he would have at least one atrocity on his record. At the time, Arthur felt obligated to obey his father -- and I think he genuinely believed that Uther was right about magic. However, Arthur is still a fundamentally decent human being, so it also makes sense that any cruelty he might've perpetrated in the past would weigh heavily on his conscience -- particularly after stumbling upon the site of one of those massacres. Everyone assumes that Arthur is so grim in this episode because he's pining over the now exiled Gwen, but make no mistake: As the ending makes clear, he was remembering what his knights did to that Druid camp many years ago.
And speaking of that ending: Perhaps it was a little too easy. Perhaps the Druid spirit shouldn't have trusted Arthur that quickly. But SABR Matt and I both found it a pleasant surprise. As SABR Matt remarked over email, "At first, I was like, 'whaaaaaaa?' But then I was like, 'AWESOME!'" At that moment, the title of the episode made sense for the first time. The "New Age," of course, is the age in which Arthur establishes Albion -- and his promise to the tormented Druid ghost is the first signal that all the prophecies in re: Arthur will be fulfilled.
By the way, I've found Arthur's arc in this series as a whole to be quite interesting. He's being pressured to emulate his father's ruthlessness, but he's also learning that there's a superior way to lead -- that his own better instincts are reliable guides and that mercy and Christian charity are marks of a truly powerful, admirable king. The only thing we're waiting for now is for Arthur to figure out that Agravaine is a treacherous beast who doesn't have his best interests at heart. Once that happens, we'll definitely be happy.
The conclusion might be slightly convenient, but I still really like this script.
The performances are also strong.
SABR Matt and I both loved seeing Arthur take responsibility for his own actions. That, after all, is what a hero is supposed to do.