When it comes to accidental killings, I divide them into two groups - the player accidentally causing a death and the PC accidentally causing a death.
Most the first one group can be handled by the GM saying "You do realise that doing that might kill someone, right?" and then the group talking things through. I say most because I've seen some unforeseeable deaths.
The second bit, when the player knows that death might occur but the character would do something like that, can lead to great role playing.
If no one can envision a death occurring then that's one thing and we can debate intent, but if the players or the GM (who should pass along a warning) can see it happening then that's another. If you know it is possible and still do it, then you've opened yourself up to the consequences. You shot into that crowded room, you brought down the ceiling, you flooded the park with water - whatever you did you knew that there was a chance of death.
Which is why Dresden shows so much restraint. There are countless times that he could unleash something major but doesn't because he knows that people might get hurt. He won't even share information if he thinks it will lead to someone getting hurt.
That's because not hurting innocent bystanders is a theme of the series. Any action you take (or don't take) has consequences and those consequences have to be ones that Dresden can live with.
Speaking of consequences, if you use sponsored magic and build up a debt, that was your choice. Maybe you'll have to steal a baby for the Summer Court or burn down a church to pay for your use of hellfire, but when you took that power you knew that there wasn't a free lunch. That you would have to pay for everything eventually. Would it be fair to those without sponsored magic if your PC moved heaven and earth then didn't have pay for it? Of course not.
Along the lines of "There's No Such Thing As A Free Lunch":
If you take an Aspect of "Kicking Ass and Chewing Bubblegum, and I'm All Out of Gum" then it will help you in fight after fight, but when that idiot pushes your buttons why shouldn't you be compelled to put him into his place?
There are downsides to all Aspects - and if there's any doubt of how it should be compelled then the Player and GM really have to get on the same page before the compels start.
Spoiler for Turn Coat:
Read the end of Turn Coat and then tell me that Morgan's actions after the traitor was unveiled isn't a great example of a compel? Morgan doesn't have to run after the traitor - and doing so could easily kill him, but there's a traitor to the White Council and Morgan has to deal with it. He can't trust anyone else to deal with it - he has to. And as an example of someone buying off a compel there's Morgan's "I didn't tell the council" speech at the end. Not telling the council goes against everything he believes in (and a couple of Aspects) but he sees the big picture and makes a tough call on a minor offense.
Richard