Robin Hood. Dick Turpin. Butch Cassidy. Bonnie and Clyde. Joaquín "El Chapo" Guzmán.
People who break the law have always been the subject of fascination, and for a certain subset of the fascinated, glorification. We still enjoy caper movies about criminals pulling off complicated heists, movies which gloss over the innocent victims of crime or even depict the criminal as an instrument of poetic justice. For the vast majority of people fascination with criminals is harmless. Living in a civilized society requires restraint that makes fantasies of anarchic behavior attractive. In moderation, some measure of admiration of rule breakers probably helps keep the people who run things in check (e.g. the Edward Snowden case).
The problem is that some people have difficulty separating fantasy from reality, keeping to moderation, or understanding how complex or ambiguous people can be. Julian Assange is neither an angel nor a devil, but a flawed, complicated person who did something that needed doing. George Washington wasn't the childhood paragon of the cherry tree legend, but an ambitious, rash, somewhat dishonest social climber who achieved greatness under the pressure of circumstance.