^ I wouldn't go that far. They provided a good conflict for Shinji: whether to help them in their common goal at the cost of his friend, or to oppose them to save said friend but let the War continue.
However, I do think that Tiger's heel turn during that arc was out of nowhere. It should've been him becoming a sort of Frankenstein's monster of Alternative's creation, carrying out his wish to end the War even at his "master's" expense.
I think there's a level of unspoken meaning to this show about mental health. Schizophrenics have a problem with mirrors and the like, not seeing yourself as who you are and so on, the whole thing started because of an ill young girl and her brother and his grief, Kitaoka has a brain tumor, Goro is a obsessive or Stockholm syndrome or something, etc. etc.
so for Tiger, he's the most extreme example of a mental break. The stress of this horrific situation caused him to stop all rational thought, and become a total sociopath, which is a wonderful counterpoint to Asskura's psychopathy. This is somewhat reinforced by the movie, wherein Yui has to accept her fate to stop her brother, and once she realizes that she's gone and he's been fighting for a ghost, he just fades off. His mental demon disappears, and so does his conflict, which was all he had holding him to existence.
Seriously though, the last two episodes of the series are so uncontrollably epic (a word I don't bandy about) that I can hardly believe it.