Not Snoke, not Rey's parents, nothing. Why did Luke Skywalker let Han Solo die?
Why are we getting sidetracked by Snoke and Rey's parents?
If they were off the playing field, there would be no Vader. Or Kylo Ren. So instead of doubling down and training NEW Jedi to take down his nephew, he simply ends the cycle.
VIolence begets violence and Luke would no longer participate.