Good and Evil
See? I told you there'd be more afterthoughts.
Part 1 is:
Part 2 is in response to an observation on a Facebook writer's thread asking how you show a Good Character turning to the Dark Side.
The discussion thread got all wrapped up in the writer's view of one specific character, but to solve the writing craft problem you need the underlying principle, not the surface decoration.
Here's the basic PRINCIPLE:
"Good" respects the Free Will and Personal Integrity of others, and will not use power of any sort to over-ride the Free Will choices of others (by lying or by withholding information).
"Evil" is so focused on gaining (whatever - money, power, relief from fear, pain etc) that the Free Will (both the power of Will and the Freedom to choose to act differently than Evil wants) of others is not important enough to make Evil hold back on use of force, and tediously explain and teach and illuminate until the other changes their mind OF THEIR OWN FREE WILL.
Evil has no patience. Good has nearly infinite patience.
Evil has no recourse other than FORCE - while Good has a life-time-created stockpile of various options.
So just show your GOOD character taking pain to avoid forcing another -- then in later scenes show that character oblivious to another's right to choose their own actions.
For more clues, read Blake Snyder's 3 book screenwriting series SAVE THE CAT!
Previous mentions of SAVE THE CAT! include:
"Saving the cat" is the description of how to formulate an opening scene establishing the main character as "good" -- someone who would take a risk to help an innocent. See the SUPERMAN movies.
First establish the "good" -- then the imperative reason that "good" has to impose his "good-ness" on another despite the resistance of the other -- then redeem your MC by showing the epiphany where he internalizes the difference in when to use force and when NOT to.
If you need more clues - read some books on Martial Arts and/or training to use a Gun. Law Officer training manuals.
In Tarot it's called THE LORD OF SHORTENED FORCE - 5-Swords.