I remember my ethics professor used the trolley problem to show us that we could argue for whatever side using deontology/principalism or utilitarianism/consequentialism... depending on interpretation and what is defined as net good. At the surface it may seem obvious what to do but eg someone who may or not have been my SO argued theoretically that human beings are awful/use up resources we're all competing for and it may do net good for more to die.

I think one must combine things: the categorical imperative or principles are useful in our everyday life because they help us make utilitarian calculations and think about wider consequences. It really is difficult thinking through all the consequences of a decision one's self, and rules/principles help us make that calculation. Also I don't think it's hugely unethical for humans to prioritise themselves and their own welfare over others and for that to enter their calculation.

Also I don't ascribe to inaction as a non-choice, your choice is to kill one person or four. The psychological make up of our brain tells us it is bad to actively participate, but it's a cognitive and emotional error.