Human breast cancer, in mice.

Yes, in mice, but human cancer cells:

"When we systemically administered our nanoagent in mice bearing human breast cancer cells, it efficiently accumulated in tumors, robustly generated reactive oxygen species and completely eradicated the cancer without adverse effects ..."

So it kills human cancer and doesn't harm the mouse in the process.

Literally reactive oxygen species targets cancer cell DNA. We are taking advantage of the unique chemical environment of the inside of a cancer cell and using it to generate oxygen in a double-whammy to destroy itself.

This is perhaps the best targeted method devised as it seems to collect basically entirely in tumors. Chemo and Radio therapy just aren't that targeted.

I agree, or at least I would stress that people should be allowed to consent to that. I don't know what the prevailing medical ethics of doing that kind of thing in consenting patients in that state, but my uninformed intuition is I would disagree with it.

Though one thing that I might think researchers might not want is people may be too sick to recover even if their cancer disappeared tomorrow.