I like all of the above answers—especially a Robin Hood type of character for fantasy role playing.
A thief in a role playing game is someone with a skill—not necessarily a criminal or acting, practicing thief. Thieving skills can be used in other ways in role playing adventure scenarios.
But any character without means or qualifications for employment, especially one in a far country, exile, or far from home, might have to steal for survival or maintain an appearance of normalcy, economics wise.
The justification would probably be that it is necessary due to temporary circumstances and that they are nonetheless serving a better purpose. Ethically it would be best to steal from where it would do the least harm or draw the least attention—or stealing from unethical persons such as the mafia, gangs, bandits, dishonest politicians, or other corrupt individuals.
An interesting twist would be someone who IS basically good but feels burnt out from a thankless struggle or lack of funds—or someone whose conscience has eroded to the point that they feel justified even though they are actually doing something wrong or criminal.