In one of Schwarzenegger's last acts as Governor of California, he slashed Esteban Nunez's 16-year sentence by more than half.
He followed the decision by writing a letter of explanation to the victim's father.
In the letter, obtained by a local San Diego, California radio network, the former Hollywood star-turned-politician writes, "It is with heavy heart that I write to you in acknowledgment that my commutation of Esteban Nunez has caused you more pain.
"I recognize that the last-minute nature of my final acts as Governor provided you no notice, no time to prepare for or absorb the impact of this decision. For that I apologize."
Santos was killed in 2008. Nunez was sentenced to 16 years in prison for his role in the incident. Santos' killer received a similar sentence.
The victim's father, Fred Santos, is now considering legal action against the former Governor.
He tells the radio station KNX, "We think that our constitutional rights as victims have been violated because we were not notified."