Those folks above who say that you should hand over the book to the accusers and challenge them "the evil" are on to something, but they aren't going far enough. Even the name of this thread contains the real problem you're having here.
"How to convince ..."? Baloney. If I have to have this discussion with anyone, I merely point out that I know a great deal about D&D, and they know nothing. I'm the expert, so if they expect to tell me it's evil, they'll need to prove it. The onus of proof is not on me, it's not on you, it's on the claimant. I only have to convince them if they're the ones who are right by default. The claimant is not right by default; so they have to convince me.
If it sounds like I'm less than diplomatic when I deal with this matter, I admit, I'm not. I spell it out and let them know until they have the proof, I don't have time to waste on their nonsense.