Both Uses of

principle
in
In Cold Blood

When she began this letter, she intended that it should be a compassionate demonstration of Christian principles.†
Chpt 2

principles = basic rules or beliefs

However few people can successfully demonstrate a principle in common ethics when their deliberation is festered with emotionalism.†
Chpt 2 *

principle = basic rule or belief

Definitions:

(1) (principle)
a basic rule or belief
The exact meaning of principle can depend upon its context. For example:
- "our guiding principles" -- basic moral beliefs that guide decisions and behavior
- "electromagnetic principles" -- rules describing how the world works
- "She lacks principles." -- lacks moral guidelines
- "We agree in principle." -- about important basic beliefs
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

See a comprehensive dictionary for more specialized senses including one in chemistry.