Merlin: The Beginning of the End Spoilers

[Sir F. A. Rien]

In this episode, Merlin and Arthur help a young Druid boy escape
execution, in spite of Merlin's dragon mentor warning him that the boy
would someday kill Arthur! At the end, the boy gives his name as
"Mordred". Since Arthur helped to save young Mordred's life, why
would Mordred kill him?

Apparently this show is taking major liberties with the King Arthur mythos.
If memory serves, Mordred was Arthur's son!

Gotta keep the interest of the thumb in mouth crowd, who cares about
'facts'?

[Sir F. A. Rien]

In this episode, Merlin and Arthur help a young Druid boy escape
execution, in spite of Merlin's dragon mentor warning him that the boy
would someday kill Arthur! At the end, the boy gives his name as
"Mordred". Since Arthur helped to save young Mordred's life, why
would Mordred kill him?

In at least one telling, Mordred is Arthur's bastard son by his half-sister[*].
[*] Mary Stewart, _The Last Enchantment_.

"Of all my 'in' laws, I like incest the best." [Pendragon]