So, what did everyone think of Optimus calling Megatron brother?
I took it as Prime said "brother" because he was a "brother transformer".
That all Transformers were Prime's brother, that they were all children of the Allspark, and therfore all brothers.
Because that is the way Prime is/was. He cared for all beings, including Megatron. Hence his decision to sacrifice himself destroying the Allspark, rather than kill Megatron and destroy the Allspark.
