It is lame it got cancelled

Morgan made this show. She made it interesting.

There's no Lancelot in this series, although there is a Lancelot-like character in that Guinevere is torn between two men.
As for Merlin's cruelty: no particular cruelty in most medieval Arthurian legends, but there are a few earlier non-Arthurian texts that portray a rather dangerous madman. In one of them, he lives as a wild man surrounded by beasts in the forest. Although he has given permission to his wife to remarry, due to his inability to remain her husband, he goes violently mad on the day of her wedding and goes into a killing frenzy with the help of the herd of wild animals he has brought to the feast. He arrives riding a deer and when he sees the bridegroom, he goes berserk, rips off the deer's antlers and stabs the groom with one of them. There is however little to nothing of that character in the later Arthurian texts. As for modern literature, everything and anything has been written about the characters so I'm sure there are some cruel Merlins out there. Bernard Cornwell's Merlin comes to mind. I wouldn't call him necessarily cruel, but he's definitely not the nicest guy either (he has no problem with human sacrifice, for example).

"Occasionally I'm callous and strange."