We could say she sabotaged it out of human decency, so the disease doesn't kill him and the 'extraction' of the bending essence thingy could be considered a failure. They've never used it on someone so young, so they may not think anything of it. There's scum and then there's child-killing scum :C
Pema's death is just so Tenzin and Lin can rekindle the old flame, but from a story standpoint it could also have been an accidental thing as well. It just depends on the level of tragedy we want to go with but I know if Meelo does die Tenzin is going into a place I don't think anyone can get him out of, not even Katara. And if you do pull him out of a slump there'll be a lot of anger. His wife is one thing, but his child is just ...I can't even do the rage justice.