Under section 105 of the evidence act the rule of burden of proof is described that the burden is on the accused to prove the existence of circumstances bringing the case within any of the exceptions "and the court shall presume the absence of such circumstances" it is not intended to displace the aforesaid additional burden of the prosecution.  It is only where the prosecution has proved its case with reasonable certainty that the court Can rest on the assumption regarding absence of circumstances bringing the case within any of the exception,this presumption helps the court to determine on whom is the burden to prove facts necessary to attract the exception and an accused can discharge the burden by "preponderance of probabilities" unlike the prosecution.  But there's no presumption that an accused  is an aggressor in every case of homicide if there is any reasonable doubt, even from the prosecution evidence, that the aggressor in the occurrence was not the accused but would have been the deceased party, then benefit of that reasonable doubt has to be extended to the accused, no matter that he did not adduced any evidence in that dissection