They are just following their leader who says to "...smite them at their necks...(4-74).
From The Straits Times July 29
Islamic militants beheaded Philippine troops: navy
MANILA - ISLAMIST militants in the southern Philippines beheaded two marines in a brutal knife attack during a major clash that left five other soldiers dead and 26 wounded, a military spokesman said on Friday.
The Al Qaeda-linked Abu Sayyaf severed the heads of two of the seven dead marines in Thursday's clash, navy spokesman Lieutenant-Colonel Omar Tonsay told reporters in Manila.
'I can confirm two,' Mr Tonsay said when asked if any of the dead marines were beheaded. 'Definitely this is a form of barbarism.' Military spokesmen in the south also raised the number of wounded soldiers from the clash on the Abu Sayyaf stronghold of Jolo island to 26, from the 21 initially reported.
Eleven of the wounded were flown to a military hospital in the southern port of Zamboanga on Friday along with their dead comrades, said the regional military spokesman there, Lieutenant-Colonel Randolf Cabangbang.
He said 30 militants were also slain, but gave no indication as to how the military determined the Abu Sayyaf death toll.
He said the government had not recovered any of the militants' bodies, even though the military reported taking control of the jungle area where the clash occurred.