What she did is essentially the same as arthas in stratholme, with the difference being she kept a level head afterwards, or to boil it down further the end justifies the means.
Doesn't really matter though, another soul would have then died in ursol's place, just from another grove.I guess some people didn't see that she said she would honor the dude's wish even if he wanted to take care of that one soul, Ursoc.
She is of course correct, doesn't make her choice any less evil though