Slippery slope

Slippery slope - a course of action likely to lead to something bad or disastrous; the potential beginning of a trend;  if we allow something relatively harmless today, we may start a trend that results in something currently unthinkable becoming accepted.

Example - "He's on the slippery slope towards a life of crime"

Image - https://blogs.thegospelcoalition.org/justintaylor/files/2013/04/Slippery-Slope.png