"Justified" ? What's justification to do with it, except of course beyond the rhetoric of the culprits ? A power struggle is a power struggle, regardless of the propaganda the participants throw around. The Roman emperors generally died of unnatural causes anyway; if I recall correctly, exactly two died a natural death...

Caesar was a man before his time and a visionary of what Rome would later be. He established enfranchisement as a way of bringing other peoples' loyalty to Rome and used soldiers from other cultures to compensate for weakness in the Roman military machine, such as his Germanic cavalry.
Uhh... and old Julius lived when ? First century BC ? Rome had been assimilating conquered peoples and recruiting them into the army since the very beginning - that's what made it possible for them to lose an army after army to Hannibal and Spartacus and other military disasters, raise some new ones and come back for more.

Mercenaries are something everyone used anyway. Caesar wasn't exactly a pioneer in that regard either.