● The requisites of fulfillment of duty are:
  1. The accused acted in the performance of a duty or in the lawful exercise of a right or office;
  2. The injury caused or the offense committed be the necessary consequence of the due performance of duty or the lawful exercise of such right or office. (Cabanlig v. Sandiganbayan, G.R. NO. 148431 July 28, 2005).

● The availability of the justifying circumstance of fulfillment of duty or lawful exercise of a right or office under Article 11 (5) of the Revised Penal Code rests on proof that (a) the accused acted in the performance of his duty or in the lawful exercise of his right or office, and (b) the injury caused or the offense committed is the necessary consequence of the due performance of such duty or the lawful exercise of such right or office. 

The justification is based on the complete absence of intent and negligence on the part of the accused, inasmuch as guilt of a felony connotes that it was committed with criminal intent or with fault or negligence. Where invoked, this ground for non-liability amounts to an acknowledgment that the accused has caused the injury or has committed the offense charged for which, however, he may not be penalized because the resulting injury or offense is a necessary consequence of the due performance of his duty or the lawful exercise of his right or office. Thus, it must be shown that the acts of the accused relative to the crime charged were indeed lawfully or duly performed; the burden necessarily shifts on him to prove such hypothesis (Yapyuco v. Sandiganbayan, G.R. Nos. 120744-46, June 25, 2012).


● As the deceased was killed while asleep, the crime committed is murder with the qualifying circumstance of alevosia. There is, however, a mitigating circumstance of weight consisting in the incomplete justifying circumstance defined in article 11, No. 5, of the Revised Penal Code. According to such legal provision, a person incurs no criminal liability when he acts in the fulfillment of a duty or in the lawful exercise of a right or office. There are two requisites in order that the circumstance may be taken as a justifying one: (a) that the offender acted in the performance of a duty or in the lawful exercise of a right; and (b) that the injury or offense committed be the necessary consequence of the due performance of such duty or the lawful exercise of such right or office. In the instance case, only the first requisite is present — appellants have acted in the performance of a duty. The second requisite is wanting for the crime by them committed is not the necessary consequence of a due performance of their duty. Their duty was to arrest Balagtas or to get him dead or alive if resistance is offered by him and they are overpowered. But through impatience or over-anxiety or in their desire to take no chances, they have exceeded in the fulfillment of such duty by killing the person whom they believed to be Balagtas without any resistance from him and without making any previous inquiry as to his identity. According to article 69 of the Revised Penal Code, the penalty lower by one or two degrees than that prescribed by law shall, in such case, be imposed (People v. Oanis, G.R. No. L-47722, July 27, 1943).