Direct Opposites:
* Life-saver: This is the most direct opposite in terms of action, but it doesn't necessarily refer to a person.
* Hero: This term suggests someone who saves lives and acts courageously, but not necessarily by directly preventing a murder.
* Peacemaker: This implies someone who works to prevent violence and conflict.
Contextual Opposites:
* Defender: This can refer to someone who protects others from harm, including potential murder.
* Guardian: This implies someone who watches over and protects others.
* Advocate: This suggests someone who speaks up for the rights and safety of others.
Ultimately, the best opposite of "murderer" will depend on the specific situation and the desired nuance.