They kill of hordes of Stormtroopers who could just as well be guys underneath that were drafted to the Imperial military that didn't want to but were forced into it. The Death Star could've been stocked full of these kinds of guys and the Alliance blows it all away...

Han killed a bounty hunter that was pointing a gun at his face and making threats against him. He knew there was only one way out of the cantina: through Greedo. If he didn't act first, Greedo would've. That's how it use to be anyway.