SAD traits like being judgemental, not trusting others, being easily offended and eventually, becoming bitter and envious are the ones that overlap with the narcissist, but for entirely different reasons and it manifests quite differently.
SA people inwardly judge others harshly because they've been overlooked so often, it's just easier to reject others in their mind first, to save the pain later on. Narcissists do it outwardly because they believe they're superior and it elevates their self esteem putting people down.
SA people don't trust others because they've been hurt too many times, trusting isn't viable anymore. Narcissists are untrustworthy, they project this onto others, therefore, no one can be trusted.
SA people are highly sensitive and embarrass easily, they find it hard to laugh at themselves and can't take criticism, even if it's well meaning. This stems back to their hurtful past, never being in the moment, ruminating about minor slights for weeks or years on end and their self pitying attitude, they usually won't lash out, they'll lick their wounds in private and the cycle continues.
Narcissists believe they're perfect and without fault, to be criticized results in a narcissistic injury and they'll retaliate. While they're usually impervious to criticism, any real or perceived slight about their intelligence can land the other person on their 'target' list, revenge will definitely be forthcoming, no stone left unturned until they've avenged themselves. 'Petty' is their middle name.
SA people can eventually become bitter, and envious of those who they perceive of having it all. They can be wrong in their assessments because they don't socialize, they easily jump to conclusions without evidence and believe the grass is greener over the fence. They very rarely speak up to right a wrong or apply for positions they're perfectly suitable for because of their negative inner dialogue, and for fear of embarrassment.
Narcissists are bitter and always envious and they believe everyone is envious of them too. They're bitter when they don't receive the treatment they 'deserve', when colleagues, friends or family receive accolades or attention, they'll ridicule and punish them, usually in public, they can't accept aging and their bitterness grows no matter how great their life is.