Tieflings were a result of several tribes looking towards nefarious powers for ways of surviving the changing world. With the boons granted by their evil patrons, several families of humans and other races began to change their shapes and forms, but the price they had to pay were considerable in return. In some cases, ancient humans pledged their cause to a fiend in return for a tiefling to be born at some point during their bloodline, which seemed like a small price to pay for a great boon in the now.
These dark pacts still haunts the tieflings as a race today, several centuries later. They are no longer feared or shunned, as they were in the past, but this is mostly because they are so numerous that they cannot be avoided - and just like any other civilized race, their hands and skills are needed to survive the terrors of the world.
In addition to this, many tieflings were also quick to understand emotions and use such insight to their advantage in conversation and politics.