It's agreed on that the duty to rescue can be waived or reduced when this would put oneself in danger. So if the victim is lying in the middle of a highway, you probably can't be expected to do a Cardiopulmonary resuscitation right in the middle of the road. But you can still be expected to call the police and an ambulance. Because that is so easy today, it's very unlikely you can ever argue that this was unreasonable.
There's a dispute over whether it's reasonable for you to call for aid if you're putting yourself in danger of prosecution. Attorneys are in agreement that this is not the case when you're responsible for the accident yourself. Here's a textbook example for that (quoted after the above link):
T ist betrunken Auto gefahren und dabei mit dem Wagen des O kollidiert. Er lässt sich von einem Freund von der Unfallstelle abholen, ohne dem schwerverletzten O zu helfen. Hier war die Hilfeleistung dem T eindeutig zumutbar.
T was driving drunk and hit the car of O. He calls a friend to pick him up without helping the severely wounded O. Here, rendering help was clearly reasonable.
The case is less clear if the wounding of O is not related to the crime of T, as in this example:
T hat drei Monate zuvor einen Raubüberfall begangen und ist seitdem untergetaucht. Während er über die Landstraße fährt, entdeckt er im Straßengraben den durch einen Autounfall schwerverletzten O. Hier wäre es T zumindest zumutbar, von einer Notrufsäule aus einen Rettungswagen zu alarmieren, da diese Hilfeleistung wohl nicht mit einem erheblichen Strafverfolgungsrisiko für ihn verbunden wäre.
T was involved in a robbery three months ago and has been hiding since. While he's driving on a country road, he finds the severely wounded O lying in the ditch. Here, it would be reasonable to expect T to call the ambulance (anonymously) from a first-aid post, as this wouldn't put him at great risk of prosecution.
Thus failure to render help in particular also applies if you're responsible for the bad state of the victim. It doesn't matter whether it was an accident or a deliberate act. Other commentaries seem to indicate that it's even more complicated and requires a case-by-case decision.