Demona's life encompasses many contradictions and many mysteries. Some will be revealed right away, others only over time, if at all.
When our story opens in the tenth century, Demona and Goliath are in love. She is a fierce and energetic gargoyle warrior. She and Hudson are Goliath's most trusted friends. But she hates to see the way Goliath and the other gargoyles are treated by the spoiled humans of the castle. She longs for the days when gargoyles lived alone upon their rookery.
She makes a deal with the traitorous Captain of the human guards and with Hakon, leader of the attacking Vikings. She and the Captain will convince Goliath to temporarily remove all the Gargoyles from the castle. While they're gone, Hakon will sack it and take all the humans away as slaves, leaving the empty castle for Demona, Goliath and the rest of the Gargoyles. it's important to see that as misguided as Demona was, she made her choices based on the love she had for Goliath and her clan.
Unfortunately, Goliath screws up her plan by refusing to take all the Gargoyles away with him. Goliath leaves with only Hudson. (The trio and Bronx are exploring the rookery beneath the castle.) Goliath puts Demona in charge of the castle in his absence, at least in part because he is protective of her and believes she'll be safe there.
The Captain tries to reassure her that the plan can still succeed. He'll sabotage his archers, and Hakon's attack can take place during the day. He promises to protect the sleeping Gargoyles.
Demona agrees, but just before dawn, she gets nervous and flees. And thus escapes the massacre.
And that's the last we see of her until 1994.
What's happened in between is currently a mystery, but it hasn't helped her disposition any. We don't yet know how she survived into the twentieth century. But we know she hasn't been sleeping for a 1000 years. Somehow, she's found a way to make herself immortal, thus she knows a lot more about the modern world than Goliath does. She's already familiar with modern weapons, technology and legalities, although at first, she pretends not to be.
She hates humans more than ever and won't be satisfied until they are totally eradicated. Her initial hope is to bring Goliath and the others over to her way of thinking. When that fails, her love for Goliath turns suddenly and completely to hate. She wants him dead.
The truth is she can't forgive herself for (or even acknowledge) her own culpability in the gargoyle massacre. She still blames Goliath for screwing up her plan. Possibly, she can't believe that Goliath could ever forgive her either. She's sure he must hate her, and wants to hate him first and more to escape that pain. In addition, Goliath's continued belief that humans and gargoyles can get along seems entirely irrational to her. She particularly hates Elisa, whom she sees as the symbol of Goliath's weakness; she may even be jealous of the detective. In addition, we don't yet know what kind of experiences she had after being isolated from the clan. But they couldn't have been very positive.
Goliath can forgive her, but he can't forget what happened. Somewhere deep inside him, he loves the gargoyle she used to be. But the gargoyle she is now is a stranger to him. Worse... she's his deadliest enemy.
She now despairs of turning Goliath and knows that Hudson would never be disloyal to his leader. She tried to guide Brooklyn to recognize the evil of humanity and join her cause, but her plan backfired and now Brooklyn holds a particular grudge against her.
In battle, Demona is a fierce and passionate warrior. She's also not afraid to use the most up-to-date human technology and weapons to fight for her cause. She has the mentality of a terrorist. Any method. Any means. But to what end? After the destruction of humanity, we don't know what she has in mind. Maybe she hasn't thought that far ahead.
No matter how she's survived into this century, she still has the same gargoyle handicap of turning to stone during the day.