Trudeau’s remarks to this effect — specifically, that we should look for the “root causes” of these events — trigger such a backlash? Nothing in the notion of looking for causes says that we cannot also deal with effects. Neither is there anything in what Trudeau said that would equate understanding your enemies with excusing them, or explaining away their acts.
The Peter Principle works thusly: in an organizational hierarchy, every employee will rise or get promoted to his or her level of incompetence. It's kind of an economic-esque model that implies a system functions based on set principles, regardless of the intentions of the people within said system. The key problem with that premise is that implies a given system is functioning.
Here's how I'd revamp the Peter Principle: people in a social system will rise (succeed) or get promoted (monetary or social status) based on:
A) their skill
B) their good fortune both genetic and circumstantial
C) their ability to take credit for the work of others
D) their ability to pass on blame for their errors or omissions to someone else.
Maybe we should call this the Paul principle - after all, robbing from Peter to pay Paul does nothing to clear the debt - it merely passes it around.