In the film trilogy, Smaug seems to be inclined to help Sauron in the war the Dark Lord is preparing for, likely out of greed, bloodlust and boredom. In these fics, he has no desire to serve Sauron or anyone else. This is because he met and fell in love with Kathryn before the events of The Hobbit took place. After allowing her to leave Erebor, he missed her so much that he spent the next six decades sleeping and talked with her in her dreams. Waiting for her to return to him was giving his life some other purpose than just guarding his treasure, so he wouldn't be so bored with his life to consider joining in a war on Sauron's side. As Gandalf tells Galadriel in An Unexpected Journey, simple acts of kindness and love are what keep evil at bay, even if only slightly in this case.
When Andraya murders King Wilhelm, why does she choose to wake him up and let him see her right before slicing his throat open? Wouldn't it be safer to just stab him with the dagger while he sleeps? Since she's framing Smaug as the murderer, she wants to make sure that the king's dead face will look like he spent his last moments seeing his killer. She knows that if Smaug decided to kill Wilhelm, he would want his own face to be the last thing the dying king will ever see. He'd also be strong enough to stab the dagger into the victim's chest, all the way to the hilt, with only one hand. By contrast, Andraya needs all of her strength to do that to the corpse, and who'd believe that a wounded maid would be strong enough to stab the entire dagger in a grown man's chest?
There could have also been some desire for a feeling of superiority involved. Andraya is already established to be a misandrist, and since Wilhelm lives up to her negative views of men through his treatment of Kathryn, it would make sense that Andraya would want to see him at her mercy right before ending his life.
Or alternatively, if Andraya stabbed her victim while he sleeps, he might cry out in pain and alert the guards before she's ready with the Frame-Up. By cutting his throat open, she renders him unable to cry for help loud enough.
Smaug is uncharacteristically hesitant about his chances to win Durin's Bane in a fight. He has heard stories about the corrupted Maiar who served Morgoth alongside Smaug's ancestors, and since the surviving Balrogs went into hiding following the War of Wrath, those stories may have come to portray them as even more demonic than they were. Dragons cannot be hurt by fire, and the same seems to apply to Balrogs (Smaug's fire attacks can't seem to actually injure Durin's Bane, only pain it), making Balrogs the only creatures under Morgoth's service who would stand a chance against the dragons. Smaug himself is two times bigger than Durin's Bane, and while most of the dragons appearing in Kathryn's vision during Heart of Ashes are only slightly smaller than Smaug, some Balrogs could have been bigger than Durin's Bane, putting them on a more even footing with the dragons whose strength has waned since the First Age. Plus, both races possess magic abilities. With Smaug having never faced a living Balrog, it would make sense he'd deem one a formidable opponent on the dragon standards from what he has heard from the stories of his forebearers.
Andraya wants Smaug to impregnate Freyja and seems confident that the pregnancy will work out. However, he later impregnates Kathryn several times. The first three pregnancies end with miscarriages, and the birth of Kolstros and Vervenia succeeds just barely, with Kathryn needing Gandalf to resuscitate her. Freyja is lucky Smaug doesn't impregnate her; she would have most likely miscarried, or died in the very unlikely scenario she would have gotten to labour. Given that she's just a teenager, even a miscarriage could have been hazardous for her health. And it's not hard to imagine Andraya would have been greatly displeased with her failure to birth the powerful grandchild she wanted and blamed her for it.