Conversation so far:
Who was the double agent?
Mystique.
Who did she turn on to become a double agent?
United States Civil Service.
Was she forced into this decision?
Professor X is forced to make Mystique his secret agent.
Was there a mission or something she went on during this time?
she attempts the theft of the Centurion weaponry from S.H.I.E.L.D.

Question: What reason did she want this weaponry?
Self-contained rewrite: What reason did Mystique want the Centurion weaponry from S.H.I.E.L.D.?