I believe a person is dumb when they do something wrong and don't even realize it's wrong. Stupid is when you do it on purpose.
Dumb: Biologically incapable to think, They can also be called as abnormal.
Should not be insulted. They do not know what they do.
Stupid: Lacking or marked by lack of intellectual acuity .Should be maltreated because they do not apply any logic in everything they do.Should be insulted.