if I think her life is in danger it is justified, if I can convince a jury that this was my belief and motivation, I don't get butt raped for life.
Your belief must still meet a standard of reasonableness that is ultimately interpreted by the judiciary, not juries. It doesn't matter if you can convince twelve uneducated hillbillies [not characterizing all juries this way, incidentally] that you reasonably believed a
woman's life was in danger because someone punched her in the abdomen and you therefore killed said someone, I promise you that will be successfully appealed if there is medical evidence to the contrary - and medical expertise will be a feature in any trial such as the hypothetical one we've laid out because, as it stands, a fetus is not a protected person under the law.
Do I think that the law needs to be extended in the way some South Dakotan legislators have decided? No.
Are there situations where a person would not be justified in using
lethal force to stop an attack on a pregnant woman where the attack is likely to kill her fetus under the current legal framework? Absolutely (though such situations appear not to have manifested in case law, which goes to show just how rare they would be).
The amendment is a thinly-veiled attempt to establish person protections to fetuses, through a very circuitous route.