The protagonist fights wrongdoers by going over what law allows. Likely believing that the law system provides insufficient punishment for the wicked and they must thus deliver it themselves.
The first video game about Vigilante protagonist was released in 1985.
Vigilantes do not only protect the "weak" (usually a side product of their activities rather than the actual goal), but often achieve this by breaking the law in various ways, regularly going over the top in their methods to punish the wicked which garners their awfully common status as dangerous criminals (which they are, really).
Batman (in some continuities), The Punisher, etc. are some good examples.