I think most people define the lines in terms of who is the aggressor and who they serve...so goblins are the aggressive faction while tieflings want to be left alone and goblins are working with the absolute, the dead three...the gods of Murder, Death, and Tyranny...possibly even Shar, the most evil deity in D&D, may be in there somewhere as her magic is in play with the tadpoles and nobody uses her magic without her permission. Plus killing the goblins can be considered just self defense here. But I think if you want to play evil you should just have fun with it, I know I will. My goal is to infiltrate the cult of the Absolute and attempt to take power at any and all costs...I am not interested in justifying my in game actions any more than the dead three are.
That's funny because based on that criteria, Mayrina's brothers are evil for trying to save their sister from a consensual pact with the hag.
