Quote Originally Posted by shinta|hikari View Post
Caster kills tons of people to suck mana from them. Looking like a bitch (which isn't even true in this case because Archer is a traitor) in front of his enemies is the last thing she should be worried about.
It's a gamble either way.

But by doing it this way, Archer has

1) gained a bit of her trust by going to her side (despite what was said this episode)
2) allowed him to be in her favour somewhat. Caster knows how difficult it is to control a servant. Trading off some ex-masters' lives for a compliant servant is a win in her books.

Caster doesn't mind looking like a bitch for her goals, but killing the two accomplishes nothing - which is why it was so easy to comply.