User blog comment:Kajune/Why did Lilith want to kill Sam?/@comment-26982641-20160403064508/@comment-3423279-20160404102346

Well, we have seen that an angel can survive being tortured by an angel blade.

Perhaps the Demon Knife can harm White-Eyed Demons, but not kill them.

Lilith also needed to break at least 65 seals before dying, and her attempts to kill Sam were more of "get stronger."

If an angel had killed Lilith the seals wouldn't of been broken correctly, and Lucifer's Cage would most likely remain locked.