Stupid Good
Stupid Good is a term derived from the Dungeons & Dragons alignment system, but can easily be applied to characters in any role-playing game in fact, it can be applied to characters in any medium, or even real life for a specific way of playing a morally good character, usually a Paladin with a relatively even split between this and Lawful Stupid.
Definition
Unfortunately, Stupid Good is considerably harder to define than Lawful Stupid, where the latter generally accepted to apply in situations where following the "Law" or pre-defined codes of conduct can lead a person/PC into morally dubious situations.
By contrast; the former would logically be applied to those who would perform good acts "no matter the cost", usually things like sparing the unarmed villain over and over again so that he can go on to commit more acts of evil in future, where a brief act of evil on the part of the good person (i.e murdering the fucker) would create an overall better outcome.
Generally though, this alignment will mostly depend on your GM's outlook, since the above example is actually considered to be one of the prime examples of the "good" alignment. This has also been spoon-fed to us over years and years of early morning cartoons where the good guy can always be relied upon to do the right thing, even to a fault and where the villain always gets away to return "next time".
Note however that this nearly always applies to situations where choice becomes a factor, the above examples nearly always depend upon an unarmed/defeated opponent. The choice of what to do with the villain then becomes a moral question, which can be entirely separate from society's requirements on the lawful/chaos axis. (where handing him to the authorities may be the lawful thing to do.) The morality issue often gets thrown out of the window if the villain is slain during a difficult fight or through circumstance.
In this case examples of True Stupid Good would be a person attempting to spare the life of a villain even then, perhaps even refusing to cause them harm since they would classify causing pain, suffering and/or injury as an evil act regardless of the circumstance.
These are the PCs who can bring gaming groups problems, when they get into the nitty gritty about what actually is classed as evil, and their strict moral codes often require them to intervene on the actions of the party members. Though anyone who owns a copy of the Book of Exalted Deeds back in 3rd Edition D&D can find that this can be a legitimate (though often extreme) attitude towards goodness, but can find rules to accommodate these play-styles.