Blood traitor

"Blood traitor, abomination, shame of my flesh! …Filth! Scum! By-products of dirt and vileness! How dare you befoul the house of my fathers!"

- the portrait of Walburga Black screaming at her "traitor" son

"Blood traitor" is a disparaging term commonly used by elite pure-blood families for a witch or wizard that sympathizes with the non-magical community and/or willingly associates with those who are not pure-blood. Other similar terms include "Muggle-lover", "Mudblood-lover" , "Dunglicker", "Scumsucker", and "Mudwallower" , all of which refer disparagingly to those who are comfortable in the company of Muggles and/or Muggle-borns, who prejudiced wizards and witches believe are somehow dirty and inferior.

Those Considered "Blood Traitors"
"You place too much importance, and you always have done, on the so-called purity of blood! You fail to recognize that it matters not what someone is born, but what they grow up to be!"

- Albus Dumbledore to Cornelius Fudge

Among those usually considered to be "blood traitors" are:

People can also be considered blood traitors by association, given that Cedrella Black was disowned for marrying a so-called blood traitor.
 * Pure-bloods who marry Muggle-borns or Muggles;
 * Pure- or half-bloods who oppose prejudice based on blood purity;
 * Pure-bloods who associate willingly and openly with those of different blood status;
 * Pure- or half-bloods who openly agree to equal rights for Muggles and Muggle-borns

Attitude Towards "Blood Traitors"
Most pure-blood wizards and witches consider themselves to be the elite of the wizarding world, akin to royalty. They look down upon half-bloods, Muggle-borns, and half-breeds, and consider the Muggle world to be inferior to their own. Pure-bloods, and even half-bloods, who do not share this view are considered traitors to their own kind, hence the term "blood traitor". They are considered disgraces by pure-blood supremecists such as the Malfoys.

Elitist pure-bloods believe that it is a sign of weak magic to enjoy non-magical company, and seem to believe that some of the supposed dirtiness of Muggles and Muggle-borns will rub off on those who associate with them, thus they consider blood traitors to be "filthy" as well.

Some extremists consider blood traitors to be nearly as bad as the Muggle-borns they are prejudiced against. Bellatrix Lestrange, for example, claimed that "blood traitor is next to Mudblood in [her] book", and her family often disowned and blasted off the family tree any members who could be considered blood traitors. Blood traitors might also be shunned by their families and acquaintances; for example, Bellatrix and her younger sister Narcissa Malfoy stopped seeing their sister Andromeda after she married a Muggle-born wizard, Ted Tonks.

Lord Voldemort and his Death Eaters targeted blood traitors during the First and Second Wizarding Wars because they opposed their goals. When they were temporarily in control of the Ministry of Magic in late 1997 and early 1998, they kept blood traitors such as Arthur Weasley under surveillance. However, even most Death Eaters were reluctant to kill pure-bloods, even "traitorous" ones, if it could be avoided. The same situation existed at Hogwarts School of Witchcraft and Wizardry while Voldemort was in power.

Muggle bias
Some wizards, go out of their way to become fans of Muggles and idolize them.

Blood traitors because of love
Some wizards are called blood traitors because they married Muggles.

Contact with Muggles
Some wizards would have been blood traitors because they were friends and were asscoaited with Muggles and Muggle-borns.