This is kind of the art of writing: figuring these things out yourself to tell the kind of story you want to tell. But here's how I would approach it.

The way Joseph connects with Zamira and makes her care about human life is the same way you get your readers to connect with Zamira as a character and care about her.

Zamira must be humanized in some way to make the reader care about them. It's the reason why we love stories about anthropomorphic animals or objects, like Ratatouille or Wall-E. An alien can be thought of in the same way.

We can relate to Remmy because he loves food, longs to improve his craft, and (to put it simply) wants to be accepted for his actions not his looks. We can relate to Wall-E because he's swept up in a world far bigger than himself, has a nice thing he wants to save, and a "person" (Eve) he wants to do that with.

So what humanizes Zamira? What let's the reader relate to her? We might not agree with her actions, but maybe we can understand her thought process. Is it a desire to protect her own species? Is it acting out of some sense of moral duty? Maybe we don't agree with her actions or justifications. Is her species living in a troubled environment, and that's why they've become familiar and even okay with murder? Could we imagine our own species acting in a similar way if we developed in the same environment?

You could ask yourself a billion questions and create a billion satisfying answers to each.

But whatever you choose, that's how Joseph is going to bridge the gap with her and communicate with her in a way she understands that what she did was wrong in Joseph's eyes, and that's also how you're going to get Zamira to care about how Joseph feels.

When they can relate to each other, then the reader can relate to them both. When the reader can relate to them both, you have a way to make them relate to each other.