Here is a spinoff question. During the time of Julius Ceasar becoming emperor where his armies would fight other Roman armies and other similar times, why were the soldiers willing to kill fellow Romans? Money was probable a big thing, but it seems like all the conflicts where just an issue of gaining political power, rather then polices like American civil war over slavery. Why where the solider willing to kill people of their same country just to have a different person in charge of the empire. Like when Rome split into East and West, if one side then wanted to retake the other would be a bit more understandable.
That's a very modern version of nationalism.
Roman citizenship was not linked to "race", family or brotherhood. It was a status you could gain for yourself. There were people who were born in Rome, spoke only Latin and were romans in every sense of the word, but were still slaves; there were people born outside of the Empire who made a name for themselves and became emperors (in the late empire at least, see Postumus).
To ask "why did Romans fight other Romans?" is akin to asking "why did nobles fight other nobles?". Fighting was the way to power within the empire, and outside of the empire, there was nothing but barbarism.
Also, human life was not valued the way it is now (or even the way it was in the middle ages). Remember this was the time of the circus and the gladiators. This way of life was not restricted to "others" like it was in the time of american slavery or even in the middle ages concerning serfs. Nobles in the middle age were born into power, and part of a separate lineage than the populace. The roman emperors, however, knew they were assisting to the games instead of participating due to their own success and strength only.
