In other words, it all fell flat when they when the writers attempted to humanize them and make them more three dimensional.

On the TV show Defiance, the main character was this cool ex-military drifter who'd adopted a little alien girl during the war. Then in seasom two, there was suddenly this backstory story where it turned out that he not only had hated her species but had murdered a whole bunch of them while yelling slurs (or something like that) and was a war criminal.

Defiance ruined his character so badly that the writers seemed to lose interest in him and his adopted daughter afterward and focused on a different set of characters instead.