If you're not familiar with something, try asking. It'll work far better then wasting my time refuting your false premises.
It is not about who needs to be killed in modern era. Both Plato and Heinlein see the primary function of the soldiers as defenders of the polis (Plato), humanity (Heinlein). For the sake of discussion we'll focus on the two works already discussed here, namely Plato's Republic, and Heinlein's Starship Troopers. Now that we cleared that up, I hope you understand that in both works the enemy is not from within, killing your own so to say, but comes from outside. In Plato's case any other Polis that decides to attack you, in Heinlein's case, the alien species of bugs that is destroying human colonies.
Military is used as an example because it is the easiest way to distinguish those who are willing to put their life on the line from those who are not in order to support the society. Heinlein for example, puts such an obstacle in place in order for the democracy to stay healthy, and not fall apart into nepotism, oligarchy, corporativism etc. To be sure, note of collectivism is strong, but it is collectivisim for the good of healthy democracy, and thus good of the humanity and its society as a whole.
It's not about fighting. It's about willingness to self-sacrifice. Unfortunatly, just liking and retweeting things doesn't make the cut.
As for your notion that we should be fighting for our constitutional rights, wasn't it Thomas Jefferson who said: "The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants."?