There are many times throughout Mary Shelley's Frankenstein where justice is seen. (One must keep in mind that the idea of justice changes with the individual.)
For some, justice is the idea that they are able to enact revenge upon another who has done them wrong. In this sense, Victor's monster was able to enact justice upon Victor for abandoning him.
Throughout the novel, the monster murders in the name of justice (alternatively, some may call this revenge). The monster feels he has been done wrong by Victor and that the only way to find justice, in his own mind, is to murder. The monster murders William and Henry in order to find justice of his own.
Therefore, a thesis statement which can be made regarding this type of justice is as follows:
While some may look at the murderous action of Victor's monster as acts of revenge, the monster was only in search of justice.