The mainstream LW idea seems to be that the right to life is based on sentience.

I don't know if this is mainstream, but IMO it's massively oversimplified to the point of incorrectness. There's plenty of controversy over what "right" even means, and how to value sentience is totally unsolved. I tend to use predicted-quality-adjusted-experience-minutes as a rough guideline, but adjust it pretty radically based on emotional distance and other factors.

killing babies is the go-to example of something awful

I think of it more as a placeholder than an example. It's not an assertion that this is universally awful in all circumstances (though many probably do think that), it's intended to be "or something else you think is really bad".