As LOMCEVAK says, this is a common problem in criminal trials. The defence raise the hypothesis that a situation occurred - self defence/misunderstanding/whatever - and the prosecution are then required to disprove it. There is no requirement for the defence to prove anything .. just to raise the matter as a "possibility". Trying to disprove a negative hypothesis is extremely difficult as the only possible way is a positive proof, and in most accident situations that is highly unllikely.