Beautiful Topic Janu di!
With my pea-sized brain.... here is what I think.
God had taken human form on earth as Lord Rama.
He did so????
Because he wanted to set an example of how a human life is ideally supposed to be.
He showed them both sides of the coin.
He showed that happiness and sorrows are part and parcel of life and how one must behave in various situations.
To all whose parents are Sanskrit students, you might have heard numerous times that Krishna Ji was solah kala sampurna.
Rama Ji however, was chaudanh kala sampoorna.
Krishna ji hid the sun in the war to save Arjun.
Krishna Ji had numerous Ranis...
Rama Ji had taken the oath of one wife.
Rama Ji never could bend facts.
He could not force Sita Maiya to change her decision to go to exile for his Raja Dharma.
He was bound.
Thus, 14 kala sampoorna.
Now, yes, I am coming to the answer.
Brahmana hatya could not have touched LORD VISHNU... He is the creator.... He is the Destroyer... He can take the life he has given.
But His Human Incarnation.... LORD RAMA was bound by principles. he needed to set an example for all human beings that what it means to kill a Brahman.
He is so pure that he cannot even be touched by anything false.
But, just to set the HUMAN CODE OF CONDUCT he followed all these rituals.