Hubris

This human quality is a driving force in both Ancient Epic literature, Classical Tragedy, Medieval Romances and Renaissance Tragedy.

In current English it translates as "excessive pride", and that is, in part, how the ancients conceived of it, but only partially, and we are interested in a broader meaning.

Another traditional definition translates as "over stepping" and usually refers to placing one's self above the Gods, or, more pragmatically, above one's cultural values and morality.

For our purposes, however, the best definition is "too much of a good thing," by which we mean an otherwise beneficial human characteristic can be destructive when in excess.

Shades of Gray: Spectrum vs. Binary
Unlike Judeo-Christian culture, which defined good and bad in simple either/or terms of "thou shalt not", the Greeks especially seem relatively comfortable living with and exploring moral "gray-ness" or ambiguity and define good and bad not so much as either/or but in terms of degrees:  "bad" can be too much of a "good" thing; thus, the immorality is in the excess, not in the thing itself.  Through the Stoics especially this "moderation in all things" will pass into Roman and then modern European culture.

Some Greek cities treated hubris as a crime -- or perhaps more accurately, they treated crimes such as rape and murder as crimes of hubris  -- and this final element helps us understand it role in tragedy especially: hubris is when too much of a good thing leads to harm.

Gilgamesh suffers from hubris by sleeping with all the brides -- while his sexuality and physical prowess is clearly celebrated and central to his definition as a hero, he's oversexed to a degree that the gods intervene to protect Uruk's citizenry.

The Greek Titan Prometheus steals knowledge, represented as fire, and gives it to mankind, for which Zeus binds him to a rock on mount Caucasus and has and eagle forever eat Prometheus' liver.  Knowledge is a good thing...unless, like fire, you have too much and get burned. 

Achilles refusal to fight the Trojans offers the most classic definition. He himself tells us that he is driven by his pride, and although The Iliad is technically an epic, Achilles' hubris has much in common with tragic heroes like Oedipus (driven by an excessive desire for knowledge) and Hamlet, who overthinks his actions.

Odysseus' men are frequently punished for their hubris, most importantly for eating the sacred cattle of the sun god, Helios, but also for unleashing the very winds that would deliver them home -- note that throughout the tale these same men are encouraged to pillage the property of others and no one is less trusting than Odysseus himself;  Homer is clearly aware that those properties which raise a man toward heroism may also plunge him toward tragedy.