Thanks for your replies.
1. I already kind of had the feeling that this idea was bit ham-fisted... I guess their Fixer will have to begrudgingly accept it.
2. Okay, it did not really occur to me the players could say no.
I don't mean that I would railroad them into accepting, it's just that we're all fairly new to SR. I guess I'll make it into an A-Team-style adventure, defending the company from another runner team and then digging up some stuff about the Johnson to blackmail him into leaving them alone.
Well, we need to be careful here because this thread seems to be the beginning of Ragnarok. But I'm gonna risk it. I'll note that I'm reading between the lines and making a few assumptions here - I'm aware of it, and if this is inaccurate please do inform me (ad if it offends I'll gladly apologize - I'm Canadian, we're polite like that), but also note that anything I'm assuming comes directly from what you've said thus far.
1. The problem isn't just that it's ham-fisted; the motivation you explained isn't really a valid one; I expect this comes partly out of a false conception of the setting. The fact that runners turning down a job never occurred to you reinforces this, in my mind. But there's more to it. See, wanting to do that suggests, to me, that you think it's your role to set the tone and impose it upon the players. The fact, however, is that the game world is a collaborative construct made with contributions in roughly equal measure from each individual at the table (the GM may make more stuff, but the PCs have, individually, roughly the same level of impact on the kind of game it is as the sum total of the NPCs - whether you want them to or not), with the rules and setting serving as the common core to keep everything together. Your players making that choice tells you a lot about the sort of tone they're looking for and the kind of runners they want to play; you should pay attention to this. And now on to more specific points.
No fixer worth his salt would have an issue with the fact that they called an ambulance for the gangers. He'd make a note of it, certainly; what this demonstrates to a fixer is that the runners aren't going to go around causing excess collateral carnage, which is valuable to him because of the value it presents to people who'd want to hire the runners. The big, loud jobs with trails of corpses in their wake are the marks of bad "runners" who are hired to create a distraction so that the real runners can work. Many of the modules would see the players rewarded for this exact sort of behaviour (especially the Dawn of the Artifacts series). Even if he has issues with these gangers, he wouldn't expect the runners to be his proxies in it - that's not their job; they are not representatives of his interests. And he certainly wouldn't go calling down assassinations just 'cause - behaviour like that gets you dead, fast. And attacking a hospital is a great way to make a lot of people hate you; this is not a reputation a fixer wants.
If you want to introduce some darker elements, that's fine - but to do it by forcibly converting non-dark elements into dark elements is the wrong way to do it. Just introduce new things that are dark elements; there's no shortage of those. If you're at a loss for ideas, though, try bugs. Look up the Universal Brotherhood.
Saving making them regret their decisions for when they make decisions they should actually regret.
2: A better way into this scenario is to have someone warn the runners that the attack is coming. That said, your difficulties here suggest, again, a setting misconception. Did you somehow think that a Johnson could ever force the runners to take a job? I'm rather curious as to what the source of this was - and it's a good thing we pointed the possibility out to you here, rather than you being unprepared for it when it happened in a session. A lot of GM's I've heard from have said they keep a couple alternate jobs prepped, just in case. Some of the modules have, as a specific point of "troubleshooting", things to do in the event the characters have objections to the job ("I don't wanna travel", and such).
And a character's background,
especially something that's purely RP like this, isn't a decision you should ever be trying to make a player regret.