Question about deontology

You might like MacAskill and Morgansen's 2021 paper addressing this very question! I'd suggest actually reading the paper, but the tldr is that they agree with you, to a point — they don't conclude that this proves "existence is immoral" but rather that traditional deontological ethics may need significant revision to handle the reality of butterfly effects and indirect harms.

Clearly consciously sacrificing a life and unintentionally setting in motion a very indirect chain of events which leads to someone dying are not the same thing, especially in deontology which cares much more about rules and principles than effects.

Frankly butterfly effects are a bigger problem for forms of consequentialism/utilitarianism, where you do care solely about ends, and are faced with the problem that not only might the utility impact of all those "butterfly effects" you cause vastly exceed the ways you try to help people, but if you choose to factor them in they also raise the prospect that whether you're a moral person or not is completely incalculable...