Why does the Necromancer let Radagast flee from Dol Guldur?

I ask this because if he captures and kills Radagast (who I get the impression isn't in a lot of contact with Gandalf or any of the wise) he can simply attack Erebor without having to reveal himself. Not only that, his top servant the Witch King was subdued by Radagast, who ends up fleeing with the morgul blade...which in return shows the wise (even though Saruman ignores it) that either the nine have returned (in movie verse) or as Radagast has saw him that a higher power is at work and the wise all know who is the lord of the nine.

So is there any reason why the Necromancer may have let Radagast simply go rather than capture or kill him and attack Erebor without revealing himself?