it just seemed to come out of left field. Ok, now I am supposed to believe this once loving person is now a homicidal zombie?
It's exactly the point of the scene. It is to seem out of place to invoke pondering.
I consider this film to be one of the best metphors of Mythology made. The Navajo Corn Pollen Path, as the final surviving crop, not from the Levant - is a superb metaphor. Joseph Campbell covered this in one of his final books before his death: http://unurthed.com/2007/05/24/the-navajo-pollen-path/
There are a variety of themes in the film on Love (not the word we use today, but a deeper Love that is tied to Mythology - not marketing for Subaru and McDonald's material world) and compassion. A lot of viewers dismiss these themes as "cheesy SciFi shit writers" but they are actually beautifully deep mythology themes that are more accurate and challenge the major problems of today (Islam terrorism, Christian Crusades in Iraq/Israel today). In particular, that Mythology originates from Fiction Book Shelves across time, and isn't supernatural in origin (which is the them of 2001/2001 films - Christian God origin).
That last scene with the fire in the field was not necessary.
It's layers deep: It is very much about the western Individual. Dante's Inferno as a personal journey for Murph, her own graduation into individual adulthood that she had been preparing for her own life. The bedroom thinking is meditation/contemplation - and ego discarding. Murph challenging her father's dream in the first scene is all where it begins, and the fights at the grade school, and even realizing Professor Brand is a using deception in his equation solving.
It's a very complex theme. But people do lie and deceive in their relationships about Love and Compassion - and it is a European Troubadour ideal to celebrate Truth against church, society, and all things ("Bigger than God and King" - year 1210 proclamation).
Murph's burning of her brother's field is a direct compassion act toward her brother's stuck ego. It is her own graduation into (symbolic age 35) adulthood.
Joseph Campbell, who was a University professor for 38 years, at age 82:
"through which a child is compelled to give up its childhood and become an adult -- to die, you might say, to its infantile personality and psyche and come back as a responsible adult. This is a fundamental psychological transformation that everyone has to undergo. We are in childhood in a condition of dependency under someone's protection and supervision for some fourteen to twenty-one years -- and if you're going on for your Ph.D., this may continue to perhaps thirty-five. You are in no way a self-responsible, free agent, but an obedient dependent, expecting and receiving punishments and rewards. To evolve out of this position of psychological immaturity to the courage of self-responsibility and assurance requires a death and a resurrection."
Again, age 35 is the same age as Dante's Inferno:
"That is the problem of Dante's Divine Comedy, too. The crisis comes in the "middle of the way of our life," when the body is beginning to fade, and another whole constellation of themes comes breaking into your dream world. Dante says that, in the middle year of his life [age 35], he was lost in a dangerous wood. And he was threatened there by three animals, symbolizing pride, desire, and fear. Then Virgil, the personification of poetic insight, appeared and conducted him through the labyrinth of hell, which is the place of those fixed to their desires and fears, who can't pass through to eternity."
That is in fact, her brother who is fixed on his fears and desires.
And why is burning a field acceptable behavior against her brother's wife/son lung cancer?
"At the very end of the Divine Comedy, Dante realizes that the love of God informs the whole universe down to the lowest pits of hell. That's very much the same image. The bodhisattva represents the principle of compassion, which is the healing principle that makes life possible. Life is pain, but compassion is what gives it the possibility of continuing. The bodhisattva is one who has achieved the realization of immortality yet voluntarily participates in the sorrows of the world. Voluntary participation in the world is very different from just getting born into it."
In terms of metaphor depth, this story is Turtles All the Way Down!