In the immediate aftermath of the war the allied powers determined the ss to be a war criminal organization making all of its members (prior to 1944 when conscription began to take place)subject to war crimes trials. So you didn’t have to be directly involved in the extermination process to be charged with something. hence the ‘accessory to murder’ charge he is facing. all of the allies and the soviets held trials for even low to mid lvl members. and after the war the respective german govts continued to prosecute former members upon discovery but most were able to evade prosecution by hiding their involvement.
So they decided guilt by association was enough.... there is no doubt of the horrors inflicted by the SS, but unless they can put this guy in some position of power where he could have directly prevented the deaths in question, this is not justice, just revenge.