He didn't patent it because he valued less the money he would make than the lives he would save and, probably to some lesser extent, the glory he would get and the impact he would make in the world (all this were only potentials at the moment of the decision).That not to mention his moral guidelines.
Patents exist to incentive innovation through a guarantee of returns over the investment. By not patenting he is declaring that, to him, the returns (not only financial) would be negative on the whole, and that he didn't need the patent system to have a drive to innovate.
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