According to the mother she “ended his living hell” …now that implies to me that he was in constant pain or something along those lines.
“brain damage” could mean just about anything.
For all close friends and family to say they feel she did the right thing implies to me that they were all very very distressed by the quality of his life.
And… … heroin and morphine are pretty closely related. As a nurse she will have known the effects of morphine and probably miscalculated when using heroin which is why she failed the first time. As for why she used heroin – getting hold of heroin from a dealer is probably easier than stealing morphine. That is just a wild guess, they don’t give us much to go on.