Why is it OK to kill a fetus because of a potential non-loving parent (some people change once the child is born) but not OK to kill a one year old with a parent who's demonstrated not loving their kid?
Because the one year old can physically survive without being inside a human being.
A foetus starts off as a single celled zygote. The majority will just spontaneously abort before anyone even knows they're there (honestly, anyone who actually believes that life begins at conception should be doing everything possible to make sure everyone uses contraception all the time to prevent any ova being fertilized at all, thus avoiding the multi-hundred-million a year death toll of spontaneous abortion). Some manage to survive long enough to become a little clump of cells, and then eventually a recognizable foetus. But until right at the end of pregnancy, they still need the environment provided by their mother's body.
The abortion debate isn't really about "killing" foetuses. Some abortion methods do directly result in the death of the foetus, but these are incredibly rare and generally only used in emergencies. In the vast majority of cases, all an abortionist does is to remove a foetus from the womb, at which point it will almost always die naturally. In the incredibly, inconceivably rare case that it
doesn't die naturally, if it shows signs of independent life, then that becomes an issue of medical ethics, but the issue is still not whether to kill it or not, but how aggressively to try and prolong its life. It's a similar decision to the one that has to be made when deciding to withdraw care from someone who is dying. Just because it is possible to keep someone metabolizing a bit longer doesn't mean it's always the best option.
So, even assuming we see the foetus as a person, the question is not "is it okay to kill someone" but "does a person have a medical responsibility to allow their body to be used to prolong someone else's life beyond the point of natural viability." Our answer, and it's a fairly consistent answer, is no.