Here is the context from the book I am reading:
Character A: Watch your temper.
Character B: Anger is relative. Others are mad enough to kill, but not I.
What exactly does character B mean by "Anger is relative?" It sounds incomplete. I know the word "relative" means "in comparison to," but obviously the character doesn't state what exactly anger is relative to.

I think character B is saying anger is relative from person to person, similar to how most emotions are difficult to objectively compare across a group of people. For example, say you replace the word "anger" with "age" as in "age is relative." A child may think their parents, who are in their 30s or 40s, are "old," while an elderly person in their 70s or 80s may view the couple as "not old at all." The definition of the word "old" changes based on one's experiences and perspectives in life, hence making the concept relative.

Going back to the context of the book you're reading, I think character B is saying that "the threshold at which one's anger becomes intense enough for them to kill varies from person to person," and since character A mentioned temper, then perhaps the comparison can be that a short-tempered person may be more prone to commit murder than a person with more patience/self-control. Basically, character B is saying that what may make others prone to kill will not cause the same reaction from him/her. The intensity of the anger others feel during a situation compared to the anger character B feels will lead to different reactions because people's tempers are different.

Sorry this reply is really wordy, I hope it's understandable and helps!