First off I'd like to thank you for being respectful and civil despite the controversiality of the topic. Continuing

But then, can something be killed if it was never born?

not born ≠ not alive. If it is alive, it can be killed. It has a beating hard, a moving diaphragm, and a brain that takes in info. All pre-birth

And what counts as a baby? It's all these aspects that I think there's no definitive answer for all cases.

Well, that's because people haven't given one. I'd like to preface my explanation with the fact that I am not religious, for me, so there's no "divinity of the soul" (or something) argument from me. It comes down to kind of an Occam's Razor thing for me.

Occam's Razor is the problem solving principle that states that the simplest solution (and/or the one that requires the least assumptions) is usually the right solution.

Does life begin when the baby/fetus/whatever leaves the mother?

Well, what about c-sections? What about the fact that the baby's physiology is identical in the moments immediately before and after leaving the mother's body (the only difference being the baby's location)?

Okay then could we say that it's alive the moment its brain starts to have a semblance of cognition? Well, it's damn near impossible to detect that, so not really.

The simplest solution to this near philosophical problem, for me, is that life begins at fertilization/conception. Or, at the very least, when it starts to develop organs. It requires the least assumptions about what is considered human.

It's very case-by-case, but I believe it should be the parents choice with some advice from doctors and the like.

Well, if the parents fucked up and got pregnant unplanned, despite the fact that most households in America are not more than 30 miles from a place that sells condoms, then it's not the baby's fault. Even in the case of rape, I believe that the baby should be aborted while it's still just a zygote or carried to term.

If the mother's life is at risk, and abortion is necessary to save the mother, then I believe abortion is justified.