User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-95.112.195.200-20150921030338/@comment-97.84.78.212-20150921043039

The colt, a demon bomb, and an angel blade could all kill Lilith.

If you don't believe that he was a seraph before Swan Song, then yes, he can now. But if you interpret him as a seraph through the the whole series, then no.