When is a Bad Character Too Bad?

Literature Devil points out that Stan Lee deliberately

made Tony Stark someone his readers would dislike.

Literature Devil discusses "Can a Racist be a Hero?"

One problem with this question, of course, is that racism can be personal and chronologically relative. In Quantum Leap, Sam gets upset when the child of the man he is currently representing uses a racial epithet. He has a visceral negative reaction, but the boy doesn't perceive the term as loaded and certainly isn't evil. In one of Jackie North's novels, protagonists react negatively to the main character using a proper term according to OUR time. To the people of the past,  however, his comment appears the product of prejudice.

Literature Devil's point is not that language is not the best guide to determining goodness or badness. Rather, characters who struggle against their own weaknesses are more interesting than characters who are already perfect. The video is well-worth watching. It concentrates on the need to create character-driven drama in which characters struggle.

I, however, am more interested in the bigger issue: How much weakness and bad behavior will readers/viewers tolerate? When does the hero stop being complex and start becoming awful?

From the "T" author list is Josephine Tey's Alan Grant. Grant is not an entirely likable guy, and that characterization doesn't appear to be a mistake. At one point, in The Singing Sands, Tey has Grant entirely miss the fact that his cousin has tried to set him up with a very nice, intelligent, and level-headed woman. He is a little narrow-minded in various matters, having certain blind-spots. Yet he is a decent investigator. 

I like Grant in part because he isn't broody, and I find the unending number of broody British detectives rather exhausting, mostly because their broodiness is NOT presented as a weakness. A better example of another good, flawed detective is Poirot, whose real flaw behind all the idiosyncrasies--his lack of humility--is what makes him memorable.

The line for when readers start to dislike a character can be quite personal. However, overall, I suggest that too-bad-to-be-liked characters don't care about the harm caused to others and never acknowledge the harm. As with King John and Richard III and Edmund from King Lear, courageous if conniving kings and sons can actually be somewhat likable or, at least, interesting. But a lack of self-awareness crosses the line. Spike as Spike was always more likable than Angel as Angelus.   

And the bad behavior doesn't have to be extreme.  

So a character with massive self-doubt can be relatable. A character with massive self-doubt who turns the entire world on end to serve the character's needs is not, especially if that character remains convinced "but my needs DO outweigh all others' needs." 

A rogue like Nick is likable and defendable--he's never really going to hurt anyone, and he faces his choices. A rogue like Wickham--who never stops playacting and grifting others--is not.