I have always thought the way the lore reads, mashing all the various versions together, that fiends either raped or had willing relations with supplicants. These created the half fiends that subsequently had children, the Tieflings. When a Tiefling has a child the "tiefling" gene is dominate and the characteristics pass on to the child even if both parents are not Tiefling. However Tiefling children are shunned by most of society as they are seen as spawn of evil. This is not however true as the inherent evil of the Demon or Devil has been diluted by the human genetics. The result is while they could be evil, nurture plays more of a role than nature.
This means if a Tiefling child is raised in a family that has direct worship or fealty to a Demon or Devil, then those evil tendencies will be nurtured and the Tiefling will likely be evil. If however the child is raised in an environment of love, community and taught such lessons, they will likely turn out to be "good".
