Not only are they not the good guys, the military started a fight where none existed in order to justify its existence.
Buenos Aires was 100% a false flag, there's 0 chance bugs in any system other than this one could have, in less than ten thousand years, encountered humanity and started lobbing asteroids at them.
Even if they had the knowledge of where humanity is from, and the ability to target asteroids in order to reroute them, they simply don't have the technology to speed an asteroid enough to be a threat to another planetary system.
The military hauled an asteroid to hit a human population center. 100%.
They knew OF the bugs before then, and although intent seemed clear, I don't believe they were at war before Buenos Aires.
It was definitely the attack that sparked the invasion and made Rico and friends give it 100% since their home was destroyed. Gotta get payback for that.
If by "previous marines" you mean the news report at the beginning of the movie?
That's the invasion of Klendathu that Rico goes on after BA gets hit. We even see the reporter giving the report from a different angle later in the movie.
No I mean people like his instructor who was already missing a limb before BA gets attacked. There are a bunch of people missing limbs before any attack.
Ah. Presumably these were lost in some other conflict, probably against other humans. We don't know of any OTHER Aliens, and since the war hasn't started yet we don't know one way or another.