All my characters swear and take the poison. Becoming a Mind Flayer is not like contracting lycanthropy or even becoming a vampire. There is no "you" after that. You just die and a new entity is born from your corpse.
My good characters take it because it is the morally right thing to do. Death is preferable to allowing your death to usher in a great evil that will harm and infect more people.
My neutral characters take it because if they are going to die they might as well go out on their own terms and prevent an evil creature from being born in their body.
My evil characters take it to spite the parasite in their head and the mindflayers who infected them for causing their inevitable death.
