A gun is just a tool. It can be used for both good and bad purposes.
Also, the NY Post article says; "To some families, it’s an outrage that a person convicted of a violent crime would be awarded an organ others so desperately need. But doctors often disagree and there are no laws prohibiting someone with a criminal history from receiving a transplant."
Elsewhere in the same article, it says; "A jury found him guilty of battery and carrying a concealed weapon but acquitted him of of intent to murder. Washington County Circuit Judge Daniel Moylan called the stabbing a case of “extreme violence” and sentenced him to 10 years behind bars."
The facts of the crime are not given, but apparently a jury did not believe he intended to kill, and since the victim did not die right away, there was no killing to prosecute.