The breeding of the astrophage to make fuel for the spaceship took so much energy that Eva Stratt paved over half of the Sahara desert, causing the weather to change in Africa and Europe. She blew up Antarctica to speed up global warming so they wouldn't die of cold before the probes could return. Stratt is playing supervillain with the Earth in the hopes that at least half of humanity would survive. After sending the three astronauts to certain death, her job is to keep humanity together for long enough to save the sun.

I think it adds a lot to her decision to send Grace away. Yes, it was cruel, but she knows what is coming for her and perhaps many of the people on the Hail Mary project. She is going to hold Earth and humanity together for decades, as people blame her for destruction and as nations in all likelihood go to war over scarce resources. Then, even if they succeed, she will probably be vilified or even executed for what she did.

Stratt is standing in the wreckage of Earth, knowing she'll live and die in infamy. She sent Grace off knowing that if he succeeded, he'd be remembered forever as a hero. She tore the Earth apart so he could save it.

Edit to add a quote:

Anger.

Envy.

Fear.

Sorrow.