Most people who prevent a shooting still get dragged through legal proceedings, now, don't they? No jury would ever, ever convict on those charges, though, and proescutions will always softball it if they bother at all. Largely because everyone agrees that mass shooters are pretty evil.
Now, using your own logic, you pipe bomb a clinic and it magically kills only the abortion-providing doctor. You are arrested, you are charged, and you are put before a jury. Do you et why your analogy is retarded yet, or do you really think an issue which sees itself as a 50-50 split is the same deal as "are active shooters good or bad?"