Calling someone with down syndrome a retard is a slur. Calling someone who's acting like a retard, aka just being stupid, slow, limited, whatever, who's not medically handicapped in any way, not disabled. They can be a retard. It's not nice to call disabled people names. People who are theoretically not disabled. Being extremely stupid, short-sighted, sometimes evil. I think that's a fair word to use. It's a slur, but it really depends who you're slurring as to whether it is meaningfully a slur that we should avoid using.