1.) What was the purpose of the Chitauri? Once they got into a fight with any professional or contractor army, their days were numbered. If they were to frighten the civilian populace, the Other's plan would have backfired. Killing civilians doesn't make them scared; it makes them mad and mobilizes them to fight against the intruders.
Well, we see at the end that they're commanded by Thanos. Generally, Thanos doesn't care who gets killed, just so long as a lot of people die. He's in love with Mistress Death and getting a fuckton of people killed is the equivalent of sending her a box of flowers.
He's the sort of guy who will indiscriminately exterminate half of all life in the universe just to impress a girl.
The most obvious thing is that he was just going through the motions so that he could get the Cosmic Cube from Loki, and possibly get a shot at the Infinity Gauntlet (if he knows that the Asgardians have it).