Updated!!!

It's certainly authentic. There's considerable number of first-hand accounts from GIs implying or outright stating that men from the Waffen SS were killed out of hand. Many of these killings were retaliatory in nature, means of getting vengeance for past SS atrocities. Wehrmacht troops also killed POWs and were killed as POWs, but the fanaticism and hatred weren't present to the same degree.

Since 1940, SS troops had brutalized Allied POWs. In May 1940, Waffen SS troopers had murdered captured British soldiers on at least two occasion. British officials were skeptical when they first heard about the killings. However, the Waffen-SS developed an increasingly brutal reputation as the war went on.

SS troops often fought hard and refused to surrender. Others surrendered only after killing or wounding Allied soldiers before they asked for quarter. Needless to say, this behavior didn't endear them to the American soldiers who'd just lost buddies.

There were also the killings of civilians in Occupied Europe like the Oradur-Sur-Glane massacre in June 1944. Then, there were the killings of Allied POWs taken during the fighting of 1944-1945.

There was the June 1944 massacre of captured Canadian soldiers by the 12th SS Panzer Division Hitlerjugend.

Then, there was the Malmedy Massacre on December 17, 1944. At the height of the Battle of the Bulge, a Waffen-SS unit gunned down 80 American prisoners near the small Belgian town. On the same day, eleven African-American soldiers of the 333rd Field Artillery Battalion were tortured and killed by SS troops in the Wereth Massacre.

Killings like these, especially the Malmedy Massacre, enraged American soldiers. On December 21, 1944, the 328th Infantry Regiment of the 26th Infantry Division issued written orders stating: “No SS troops or paratroopers will be taken prisoner but will be shot on sight."

In what seems to have been a direct response to Malmedy, GIs from the 11th Armored Division killed some 80 Wehrmacht POWs on New Year's Day, 1945 near the Belgian town of Chenogne, Belgium.

There were a spate of other shootings as well. On April 15th, 1945, the 254th Infantry Regiment of the 63rd Infantry Division executed between as many as 30 Waffen-SS and Wehrmacht POWs in Jungholzhausen, Germany. Days later on April 22nd, 1945, tankers from the 12th Armored Division killed 24 Waffen-SS prisoners of war.

In fact, Waffen SS men were being killed with such frequency that they had to be reigned in. Peter Schrijvers writes about the 90th Infantry Division in the Saarland, which "executed Waffen-SS prisoners in such a systematic manner late in December 1944 that headquarters had to issue express orders to take Waffen-SS soldiers alive so as to be able to obtain information from them."

Understandably, the liberation of concentration camps excited further killings. In The Liberator, Alex Kershaw writes about what happened to some of the SS troops and remaining SS Totenkopfverbande guards defending Dachau. Given what the GIs saw, what they did wasn't surprising:

Many of the victims [of the camp] were naked. Some had been whipped. In one car, a dead man lay on bodies, his face frozen in agony, having cut off his grangrenous leg with his own hands. The stump was covered in dirty paper. There were thirty-nine boxcars in all, containing some two thousand corpses. The train had left Buchenwald with around forty-eight hundred prisoners some three weeks earlier...when the train pulled into Dachau, there were just eight hundred. The dead were left to rot on the train.

[Lieutenant Colonel Felix] Sparks spotted two dead inmates on the ground nearby. They had been killed in the most bestial way. All he could do was stand and stare. Then he vomited. One of the victims had had enough strength to crawl out of one of the boxcars. But then a German had crushed his skull with a rifle butt. His brains were spattered all over the ground.

Sparks's men also saw the corpse with the crushed skull. Some began to cry. others cussed. Most were dumbstruck. They couldn't believe what they were seeing. It was unreal. Maggots. The smell. Naked people stacked in heaps...

Disbelief turned to rage.

"Take no prisoners," someone said.

"Let's kill every one of these bastards."

"Don't take any SS alive!"

...some of [Sparks's] men were boiling with rage, eager to avenge the SS crimes. I Company scout John Lee had never seen his fellow Thunderbirds so unhinged. Sparks heard men screaming and cursing. "Let's get these Nazi dogs." It was all too much. His men were losing their minds... "Every man in the outift who saw those boxcars," recalled one of Sparks's men, "felt [like] meting out death as punishment to the Germans who were responsible."

[One prisoner] saw four SS guards making a run for it ... other inmates swarmed around the fleeing SS men, one of whom elbowed a prisoner out of his way. The inmates then attacked en mass and the SS men were killed, in all likelihood stamped to death. Elsewhere in the camp, victims suddenly became victimizers and in several instances beat other SS men, their Kapos, and informers to pulps with fists, sticks, and shovels... The Thunderbirds did little to intervene, turning their backs on two inmates beating a German guard to death with a shovel.

Sparks and other officers organized the roundup of SS prisoners and assembled them in the camp coalyard. Nearly a hundred SS men were lined up against the yard's wall. When Sparks left, the rage against the SS guards boiled over.

Among the GIs in the yard was Bill Walsh, a young infantry lieutenant who'd been so enraged by the sight of the boxcars, Sparks had to briefly relieve him of command. Walsh issued a fateful order: "Let them have it." At least 17 SS men were instantly killed and up to 75 more wounded by the fusillade. GIs elsewhere in the camp even went so far as to shoot the SS guard dogs.