First of all, you should decide whether you want them to take the run or not. That would be very awkward (and hard to prepare for) if they do the opposite.
Second, I agree with previous posters indicating that indiscriminate, seemingly random killing makes little sense. You can make bigger, more attractive distractions with a lower body count (bomb threat with proof that it's legit, taking hostages and threatening to execute 1/minute, etc.).
I think that most of the sentiments expressed thus far are in line with my opinion on this, namely that you can create far darker quandaries than "more money = looser morals?".
For example, imagine the runners accepting a straightforward run where they need critical information from a gang leader to proceed. He will tell them in exchange for them capturing someone he's interested in. The runners find the person and she tells them that she was kidnapped by the gang and been a sexual slave for the past two years, and only just now escaped. She pleads for her life and freedom, saying her treatment would be even worse if she was returned.
Bam. Now they're in an awkward spot. They committed to completing the mission, but had no idea this would become a key piece of it (neither did the Johnson, that helps make this harder). They can't continue without doing it (unless you let them). But it pulls the heartstrings of slavery, nonconsent and kidnapping all at once, and IMO is much more interesting than "shoot baby for 50,000¥ yes/no?".
If you really want to pull the "shoot a child" card, I'd go for something like this. Runners infiltrate an Aztechnology arcology to retrieve a package. Once inside, a kid (say, eight years old) notices runners that he's always loved watching on the trid and tells them about a maintenance shaft under construction that leads them to their target safely. On their way out, they notice that the child and their family has been round up to be sacrificed for the kid's assistance, which was caught on camera. (to make it even worse, you can have the child refuse to give up what information he told the runners so that security can't hunt them down)
Do the runners break a perfect run to help the child? Or do they walk away callously?
The best dilemmas are the ones where you've already got some "skin in the game". Telling them straight up at the beginning that they need to do something very bad in exchange for money typically leads to either straightforward "sure let's mow them down" or "no way!", which is brief and uninteresting. Flipping things during the run can force them to do something far darker than they would under unstressed circumstances.
