Misunderstanding?
While the Martians, once they get going, are pretty "bad-ass"... a few other thoughts...
I'll be accused of taking this too seriously, but bear with me...
* The translator didn't work. That was painfully obvious. Considering what a mess it made of Martian, it probably translated English back into Martian even worse. Note the puzzled expressions of the Martians when they hear the translation.
* Prof Kessler mentions a probe recently sent to Mars. This could be interpreted as a weapon.
* The Martians watch Godzilla, Dukes of Hazzard and other stuff, probably giving them a stilted vision of what Earth is like.
* They hate birds, which is probably why they fried the pigeon, and hate the presidential seal. They probably saw this as hostility.
* When they turn up for the first time on Earth, they're surrounded by tanks, soldiers and generals. All hostile.
* They presumably don't trust the president, because they think he double crossed them.
* When the General salutes, tries to shake hands with them, and draws a circle in the air, they don't know what the hell to make of it. The circle thing could mean a planet etc.
So despite all the crap, they bring over... they could have genuinely come in peace...
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It's not "sci-fi", it's SF!