I find it difficult to believe that a character who started out good, is now serving a demon lord who is quite clearly asking him to commit evil acts. Unless this demon is some how able to convince the paladin that he is killing/sacrificing evil people, it just seems a bit shallow to me. I like to think that evil characters, like all characters, don't consider themselves to be evil. No human willingly serves an evil master, unless they believe that their master isn't evil.
I also question the motivation of the Demon Lord. I know many DM's kind of play a Demon Lord as a cliche villain with no real motivation, other than being evil, just because. And I always find that to be weak writing. What does this Demon Lord hope to accomplish? And why does he need the paladin specifically to achieve this goal?
Airen - The Goddess of Chaos
To give you an idea of different way to approach this, I'll give you an example from my own campaign. I have cultists who worship the evil Goddess of Chaos, Airen. But to them she is a harvest Goddess. Because when ever good people pray to their Goddess of the Sun for a good harvest, it means there is drought somewhere else on the world. So to the followers of the Goddess of Chaos, the Goddess of the Sun is the evil one, because she brings drought. The Goddess of Chaos upsets this balance, but brings her cultists prosperity in the process. So to them, she is the good one.
Teehlyian'tara - The Lady of the Flesh
Another example is an evil entity known as Teehlyian'tara, The Lady of the Flesh. She is a godly being who corrupts everything. Like a plague, she festers where she can, and like a swarm of roaches, her essence is always spread among various vermin. To manifest her evil powers, she needs a body, preferably a woman. So what she does is she corrupts the body of a strong woman, who then becomes her avatar on the world. But she has followers in the form of cannibals, who believe she bestows great power upon them, and elevates them to godhood. These cannibals have driven many other tribes to flee the islands, but the cannibals believe they are the superior race who are meant to ascend to gods. They reject the gods of the tribes that they crush. They are zealots basically, but due to the cruel being they worship, this translates in all manner of unspeakable acts to worship her. Their goddess embodies corruption of the body, and thus her followers do the same to their own bodies (scarification) to be more like her, and to their foes, by making mockeries of their bodies after killing them, and wearing their remains on their clothes. The cannibals know their acts are cruel, but they believe they are the good guys who are meant to become as gods. Their deity is good to them, and grants them great gifts.
Villains and their cruel masters have motivations
Teehlyian'tara doesn't just want to corrupt people because she can. Corruption is the only way she can manifest her powers on the world. She is a weak evil being, who would not survive a direct confrontation with the established pantheon of gods. So like the vermin she is, she festers and corrupts the source of power of the other gods, allowing other stronger evil beings to do the dirty work for her. She is cowardly, but through her acts other far more dangerous beings are able to seize power. Like a roach, she does not seek to rule. She doesn't even have a body of her own! It is simultaneously her strength and her weakness that she is always spread like an infestation, and can never truly be eradicated.
So what is the end goal of this demon lord? What does he hope to achieve? And why would your paladin follow him? Is there any illusion in the mind of your paladin that he is doing good deeds? And if he knows that is committing evil, why is he doing it?
