You could be saving a rapist, you could be saving the next Hitler, etc. But you could also be saving the next scientist that would revolutionize medicine saving billions, or combating climate change saving billions of humans and animals, etc. Equally well, your pet might contract rabies and kill someone, or might save a child from being attacked by another dog, etc etc. This line of thinking quickly exhausts itself of any reason, as probability goes every way. So to answer the question of "why would anyone save the stranger," we have to dig into philosophy. A generally prevalent argument in philosophy for eating animals or for doing research on lower mammals is that humans are prioritized because they're rational. It doesn't seem very convincing at face value, but there are books on the matter that you could read.

This is honestly an easy argument cause humans are just awful and the worst thing to ever happen to this planet.

Well by that argument, what's to stop you from just killing every human you come across if you can evade legal repercussions? And if no rational being truly existed on this planet, then what is the definition of "worst"? It's a bona fide relative term by all definitions. So worst for what? For beings that do not comprehend their own demise? I'm aware this comes off as psychopathic, but philosophy does take you to often uncharted moral waters on hypothetical scenarios.

I think such questions never have a black and white answer, pretty much like most philosophical questions. And your stance might very well be supported by many scholars. But an independent fact stands, your tone and argumentative approach clearly indicates a certain degree of misanthropism. Which I'm not saying is good or bad. I'm saying you're proving the point of my previous comment.