Are there really any souls that the Devil doesn't want?
Nope. He wants to lead all souls to hell.
The Devil does hate to be around people who have great faith in God and do their best to love and serve God and others.
Is it actually spiritually dangerous to live a lukewarm life (where you don't sacrifice to love God and others)?
God rejects those who are lukewarm: “I know your works; I know that you are neither cold nor hot. I wish you were either cold or hot. So, because you are lukewarm, neither hot nor cold, I will spit you out of my mouth.” – Rev 3:16-17