Here's how I could see Leia falling for the dark side:
Leia finds Kylo and senses Ben in him. Rey doesn't and strikes him down. Leia filled with rage of losing her son, starts using the dark side of the force (shoots lightning etc) and battles Rey, accusing her killing her son and that he turned good again.
Another scenario I can think of is Leia blaming Snoke for turning her son, and while she tries to kill him, she taps into the dark side of the force, not realizing it until it's too late.
Neither of those scenarios are too likely, but just shows that it's still possible to have Leia fall to the dark side
I don't think it works like this for real people (and i know we are talking about movie characters).
Everyone of us had a moment (or more) in his life when he lost his shit or did something bad on purpose not caring for the consequences for others (i assume). Did that put us on the path to be murdering psycho killers?
Star Wars never was, and probably never will be, a nuanced psychological piece on morality and its repercussions. It paints in broad and easily understandable strokes, it uses clear black and white distinctions because it is space fantasy and not a treatise on the human condition.
Just because a Jedi dips his toe in the water, metaphorically speaking, doesn't mean he has to be sucked in completely. This absolute thinking that Obi Wan accuses Anakin of also applies to Jedi themselves, sometimes i had the feeling the Jedi would like to put you away if you ever have plucked the wings off a fly just because you could.
In the now defunct EU novels there was at one point (and maybe in other novels too) the son of Han, Jacen Solo, who was a Jedi at that point and was captured/tortured by a Jedi who worked for the enemy (but had her own agenda).
The enemy Jedi didn't try to put Jacen on the path to the Dark Side but opened his mind to the possibility of there being more than absolute Black or White, i.e. all kinds of shades in between and that it was up to the person itself to choose his own way and decide for himself if his action were justifiable.
It said that it was ok to feel anger and to use it because if you were a good person at the core then you would not be affected.. this was the concept of Grey Jedi i believe and sounded much more realistic to me.