Tears of the Sun

Turmoil erupts in Nigeria following a military coup d'etat, which sees the brutal murder of the president and his family. As foreign nationals are evacuated from the country, Lieutenant A.K. Waters (Bruce Willis) and his U.S. Navy SEAL detachment Zee (Eamonn Walker), Slo (Nick Chinlund), Red (Cole Hauser), Lake (Johnny Messner), Silk (Charles Ingram), Doc (Paul Francis), and Flea (Chad Smith), aboard the aircraft carrier Harry S. Truman, are dispatched by Captain Bill Rhodes (Tom Skerritt) to extract a "critical persona," one Dr. Lena Fiore Kendricks (Monica Bellucci), a U.S. citizen by marriage. Their secondary mission is to extract the mission priest (Pierrino Mascarino) and two nuns (Fionnula Flanagan &amp; Cornelia Hayes O'Herlihy), should they choose to come.
The mission begins as planned. Waters tells Dr. Kendricks of the company of rebel soldiers closing on her hospital and the mission, and that the team's orders are to extract U.S. personnel; however, Kendricks refuses to leave without the patients. Waters calls Captain Rhodes for options; after their short and ambiguous conversation, he concedes to Dr. Kendricks that they will take those refugees able to walk. She begins assembling the able-bodied for the 12 kilometres (7.5Â mi) hike; the priest and the nuns stay behind to take care of the some injured. Irritated and behind the schedule, the team and the refugees leave their hospital mission after daybreak.
At nightfall they take a short break. Guerrilla rebels rapidly approach their position, and Waters stealthily kills a straggling rebel. Dr. Kendricks warns Waters that the rebels are going to the mission, but he is determined to carry out his orders, and they continue to the extraction point. When they arrive, Waters' initial plan becomes clear: the SEALs suddenly turn away the refugees from the waiting helicopter. Waters forces Dr. Kendricks into the helicopter, leaving the refugees stranded in the jungle, unprotected against the rebels. En route to the aircraft carrier, they fly over the original mission compound, seeing it destroyed and all its occupants murdered, as Dr. Kendricks had predicted. Remorseful, Waters orders the pilot to return to the refugees. He then loads as many refugees as he can into the helicopter instead and decides to escort the remaining refugees to the Cameroon border.
During the hike to the border, using satellite scans, they discover the rebels are somehow tracking them. As they escape and evade the rebels, the team enters a village whose inhabitants are being raped, tortured, and massacred by rebel soldiers. Aware of having the opportunity to stop it, Waters orders the team to take down the rebels. The team is visibly shaken by the atrocities they see the rebels have committed against the villagers.
Again en route, Slo determines that a refugee is transmitting a signal allowing the rebels to locate them. The search for the transmitter reveals the presence of Arthur Azuka (Sammi Rotibi), surviving son of late President Samuel Azuka, which they realize is the reason the rebels are hunting them. Samuel Azuka was not only the president of the country, but also the tribal king of the Igbo. As the only surviving member of this royal bloodline, Arthur is the only person left with a legitimate claim to the leadership of Nigeria. A newer refugee picked up during the trek is discovered with the transmitter on his person. He attempts to run but is shot. Waters is angry at Dr. Kendricks, who knew, yet never informed him.
The team decides to continue escorting the refugees to Cameroon, regardless of the cost. A fire fight ensues when the rebels finally catch up with the SEALs, who decide to stay behind as rearguard to buy the refugees enough time to reach the border safely. Zee radios the Truman for air support; two F/A-18A Hornets take off and head for the fire fight. The rebels kill Slo, Flea, Lake, and Silk. Waters, Red, Zee and Doc are wounded, but direct the fighter pilots on where to attack. Arthur and Dr. Kendricks are scrambling to the Cameroon border gate when they hear the fighter jets approach and bomb the entire rebel force.
Waters, Zee, Doc, and Red rise from the grass as U.S. Navy helicopters land in Cameroon, opposite the Nigerian border fence gate. Captain Rhodes arrives and orders the gate open, letting in the SEALs and the refugees. A detail of U.S. Marines then escort the SEALs to the helicopters. Captain Rhodes promises Waters that he will recover the bodies of his men. Dr. Kendricks says farewell to friends and flies away in the same helicopter with Waters.

Question: Who kills a straggling rebel?
Answer: Waters