Not all mad scientists are evil, although most of them are eccentric by normal human standard as they tend to be obsessed with their own creations and theories and often have their own skewed systems of morality. The majority of mad scientists believe that their intellectual prowess makes them morally superior to other people (and sometimes each other). They tend to live outside mainstream society, interacting with as few non-Mad people as possible, although many will hire assistants who will aid them in their projects.
The ability to practice mad science is hereditary and has been passed down several family lines. Some mad scientists have noticed this and have deliberately attempted to breed their own kind with mixed results.