But they expected to die yesterday and were very willing to die (as well as murder). By chance, they are both alive. If they survive they will spend decades in prison. They left home yesterday to kill kuffars and sacrifice their lives.
If they had sneakily murdered and hid, expecting to get away with it, you might have a case for saying they are cowards, but that's not what happened at all. They nonchalantly waited for armed Plod and their own probable demise. It seems they hoped to take one or two Plod with them, but they showed absolutely no sign of wanting to escape or survive.