The creature, in Mary Shelley's Frankenstein, could be seen as both a victim and an aggressor. Essentially, the denotation as either is up to the individual reader and their engagement and understanding of the creature.
On one side, the monster is outright responsible for the murders of William, Clerval and Elizabeth. As a result of the murders, one could state that the creature is also responsible for the deaths of Justine and Victor's father--given they were all the consequences of the initial murders.
On the other side, one could say that the creature was not responsible for the murders. He did not plan on murdering William. Instead, he recognized William's innocence and wished to educate the boy. The creature used the murder of Victor's loved ones as revenge.
I could seize him, and educate him as my companion and friend, I should not be so desolate in this peopled earth.
Once the creature found out that William was a Frankenstein, rage overtook him. He, essentially, could not control his rage with Victor, and William payed the price with his life. It was not until after William's death that the creature realized the power he possessed.
I exclaimed, ‘I too can create desolation; my enemy is not invulnerable; this death will carry despair to him, and a thousand other miseries shall torment and destroy him.'
One could readily argue that the creature's murder of William was not planned and, therefore, the creature was not necessarily responsible.
Ultimately, one could look to the creature's words alone to define his responsibility for murders.
I left the spot where I had committed the murder.
Here, the creature readily admits to committing the murder, with no feelings of guilt attached. Therefore, if based upon the creature's actions following the murder of William, one could justify that the creature is completely responsible for the murder.