He seems hardened, but then he has to be. The Red Cross hospital in Kigali is overwhelmed with hundreds of patients from the civil war, and more arrive every hour. There is not enough of anything to go around, including skilled people like him.
'It's worse than playing God. It's like playing Satan,' said Dr Sundin, wearing a flak jacket because of mortar bombs that land at the makeshift hospital. 'But that's Kigali these days. It's an evil place. Here we lose a lot and save a few.'
Dr Sundin, 44, an American Red Cross volunteer, is one of nine doctors working up to 16 hours each day, seven days a week, in government-held central Kigali. The capital is largely surrounded by Rwandan Patriotic Front rebels who for weeks have laid siege with mortars, rockets and artillery shells.
Doctors and UN troops yesterday began evacuating the 300 most seriously wounded of the hospital's 600 patients to a hospital in rebel-held territory.
''We're all burnt out, irritable, we'd go berserk if we had the energy. You get woken up by machine-gun fire at five in the morning, so you don't get much sleep. You can't function in these circumstances for very long,' Dr Sundin said. He has been patching up the wounded since the war began in April.
The hospital buildings once served as a Roman Catholic convent and school. Then the Red Cross commandeered them, and Dr Sundin and his colleagues immediately had to deal with scores of wounded, many of them Tutsis mutilated by government-trained militias. Most of the estimated 500,000 killed so far have been civilians targeted by Hutu gangs.
'We saw plenty of machete wounds in the first month: hands cut off, tendons clipped. They usually go for the hands, feet, head and neck,' Dr Sundin said.
In recent days, as the Tutsi-led rebels intensified their barrages, the hospital has swelled to overflowing with civilians and soldiers. Now the rows of injured, some on stretchers, many on the ground, extend into every available room and tent.
Recent arrivals are laid on the dirt driveway outside the hospital's main gate. Beyond it are another 40-odd people, suffering terrible wounds, waiting their turn for the emergency room's six beds.
'We have 80 units of blood in that fridge, but most of what we deal with is arms, arms and legs - big soft-tissue injuries that don't need blood, just time and clean conditions. Unfortunately, we have neither. said Dr Sundin.
'That boy was shot clean through the head, but behind the jaw. He'll live. That man there, on the other hand, his bullet's higher up, more serious. That's why he's over in the corner. They don't survive.'
The intensive care unit nearby is packed with bodies. 'We have 30 patients today who need abdominal surgery. We have time to do four. Picking between them is the hardest part of what we do,' he said, pointing to one bed. 'That guy, his intestines are all hanging out. There's no time to put them back in. It would take two, three hours, and I have several patients whose intestines are in. So he'll die. You just have to pick those who have the better chance of survival.'
Up a flight of stairs, the first tent on the right is the paediatric ward: about 20 boys and girls, some only babies, many without parents, all suffering some wound from the war. Despite her missing right leg and the frightened sobs of those around her, one little girl smiles and manages a bright-eyed 'Bonjour' to a visitor.
Across the way, six tents erected on tennis courts hold injured people crammed almost on top of each other. The dead pile up beside one tent.
Dr Sundin points out a small black crater, marking where a mortar bomb landed a few hours before, about five feet from one of the tents. 'It's a miracle nobody was killed by that, but then these people are half dead already.' Another bomb blast earlier in the day about 30 yards away killed a Red Cross worker and wounded three other people.
Back at the gate, two militiamen's cars - their exteriors smeared with mud as crude camouflage, their doors ripped off to allow speedy access - deliver fresh victims.
About 300 patients arrived on Friday. The weekend brought 200 more. The rebels, meanwhile, are taking their time to capture the heavily defended capital.
'It can't get any worse than this,' said Dr Sundin, surveying the carnage around him. 'Can it?'