I just wanted to put this out there in case anyone is confused:
Alexia is a god. Luca is not. They share the same father, but they both have different human mothers. Alexia got the godly gene, Luca didn't.
In my book, gods have different power levels. The original gods are the most powerful. Gods like Alexia's father. The more children are born, the more the bloodline gets diluted, and the children are born with less power. A more powerful god is capable of killing a lesser god. That is why Alexia can kill Selene, and Damien can kill Alexia. And that is why Alexia is looking for a weapon strong enough for a human to kill an immortal.