The orthodox view of most religions is abortion is a sin. One thing that does not make sense to me about this is pretty much everyone does not think curing cancer is a sin, even though a fetus and cancer seem equivalent in every meaningful way, at least from a religious person's point of view.
Both a fetus and cancer are parasites (fetuses literally use viral proteins to latch on to their mother) that will kill their host and their siblings (other cancers in cancer's case) without any sign of remorse if it seems beneficial to them. Thus, they both put others in mortal danger.
Both are human by descent and genetics.
Both can survive outside their first host and lead fulfilled lives, as demonstrated by HeLa and CTVT.
Since cancer is immortal, if you believe in divine intervention, there will inevitably be a time when it is healed to become at least equal to a modern human if it is kept alive. Thus, both fetuses and cancers will grow into a "full human."
There is the self-defense case for killing cancer, but since cancer may not even be aware its host is alive, just like the host is often unaware of the cancer's life, the real intentioned first attacker is the one killing the cancer. Also, if the cancer is removed from surgery it can be saved like HeLa and be kept alive.
Additionally one could make the distinction that Hinduism only specifies sin for embryos, but that would just raise the question reason for this seemingly arbitrary distinction. Also, Hinduism technically never states that killing someone outside the chatur varna is a sin, maybe for this exact reason. That still does not answer why killing an embryo is sinful when it seems to be the exact same thing as killing a cancer.
So has any guru or scripture explained the discrepancy?