Was it ever confirmed canonically that Itachi was a pacifist? Was it ever confirmed officially that Itachi was a pacifist? Narutopedia says the following and I wondered where they got it from...


  Itachi was the first child born to Mikoto and Fugaku Uchiha. His early
  childhood was marked with violence: when he was four years old, the
  Third Shinobi World War waged and he witnessed first-hand many of the
  war's casualties. The death and destruction he experienced at such a
  young age traumatised Itachi and made him a pacifist,
For me, not entirely.

If we base our definition of pacifist or pacifism from Wikipedia, we can read the following: 


  Pacifism is opposition to war, militarism or violence.


Yes, Itachi was indeed opposed to war and this was confirmed canonically. In Chapter 400, Madara explains that after Itachi witnessed the Third Great Ninja War:


  That trauma made Itachi into a conflict-hating, peace-loving man.


However, if we look again into a more detailed definition of Pacifism:


  Pacifism covers a spectrum of views, including the belief that international disputes can and should be peacefully resolved...


In the same chapter, Madara revealed that the Third Hokage tried to resolve the dispute peacefully but time ran out. Itachi, however, never did. He had already decided that wiping out the entire Uchiha was the only way to prevent another war.


  ...rejection of the use of physical violence to obtain political, economic or social goals...


Itachi, even though he was forced, massacred his clan in order to try to preserve peace.

So, while he might be a 'conflict-hating, peace-loving man', he is not entirely opposed to using violence to preserve peace. This conflicts some beliefs related to pacifism so with this, I do not think he is entirely a pacifist.