Did Walter become Millennium's servant by choice? Walter tells Victoria that he was brainwashed and forcefully turned into Millenium's minion, but then says that he attacks what remains of Hellsing by his own free will. Bad characters usually mess with good ones, so it's not clear how it really happened to him.

Related: Did Walter become Millennium's servant by choice? @ SciFi &amp; Fantasy
From the Hellsing Wiki:


  It is also currently unknown whether or not Walter's brainwashing by Millennium was consensual, or if he was brainwashed against his will.


However, on the same page, it is also stated that


  In chapter 75 of the Hellsing manga, Sir Hugh Irons, the acting leader
  of the Convention of Twelve, comes to the conclusion that Walter had
  in fact been betraying his superiors for quite some time, even before
  the second coming of Millennium and possibly even since the days of
  the World War II. He mentions that he foresaw Richard Hellsing's
  attempted coup of the Hellsing Organization and also Richard's
  assassination attempt on Integra, and had quite specifically warned
  Walter of it, charging him with protecting her; and yet, despite his
  warning, Walter was nowhere to be found on that day. Therefore it is
  suggested that Alucard's awakening was also orchestrated by Walter;
  his convenient absence left Integra with no other choice but to
  release Alucard, whom Arthur Hellsing (Integra's father) had
  purposefully sealed away, thinking him to be too dangerous to use as a
  weapon. Other bits of evidence suggesting that Walter is a traitor
  include the Valentine brothers' mysterious knowledge of the location
  of the Hellsing mansion, Walter's suspicious memory lapse when
  Millennium resurfaces (he does not seem to remember storming and
  destroying Millennium's base in World War II, but claims that he does
  after Alucard prods him), and the strange omission of the fight scene
  between Walter and his apparent rival, the Captain. It may be that,
  after seeing Alucard's abilities during World War II, Walter wished to
  destroy him to prove himself and thus allowed Alucard to awaken.


It's still shrouded in mystery, however, I think he wanted to battle against Alucard in a fair fight, more than "betray" Integra or Hellsing.

He may have been brainwashed and he may have not, but it was very clear that he wanted to beat Alucard in battle, and that the deal he made with Millennium was made to achieve that goal.