Unless one has access to memoirs of Russian infantryman and is fluent in Russia, its rather hard to come across individual accounts. We do, however, have many open source and translated accounts of the liberation of the Treblinka death camp from Soviet writer Vasily Grossman.

The article The Hell of Treblinka was so compelling and descriptive in the horrors and set-up of the camp that it was put forward as evidence for the prosecution at the Nuremburg trials. Disgust, disbelief and revulsion are evident in Grossman's writing:

None of these beings was in any way human. Their distorted brains, hearts, and souls, their words, acts, and habits were like a caricature—a terrible caricature of the qualities, thoughts, feelings, habits, and acts of normal Germans. The orderliness of the camp; the documentation of the murders; the love of monstrous practical jokes that recall the jokes of drunken German students; the sentimental songs that the guards sang in unison amid pools of blood; the speeches they were constantly delivering to their victims; the exhortations and pious sayings printed neatly on special pieces of paper—all these monstrous dragons and reptiles were the progeny of traditional German chauvinism.

From the third and fourth page of the linked .pdf

Grossman's experience should in no way be taken as a blanket, emotional response from all the Soldiers who were present at Treblinka's liberation, but as a war correspondent he offers a peek into the emotions and views of the men he was with at the time. Like Ernie Pyle and others, we can get a mental collage of what soldiers might've experienced and felt through their writings.

As an aside, Grossman was an avowed Communist, and his break/disenchantment with the political system actually begins, according to Beevor over this:

The Russians were not known for being friendly to Jews either

Seeing the hell of Treblinka and similar camps, then discovering that Stalin's government had a disinterest and institutional bias against Jews had a profound negative effect on Grossman.

I could only find recollection of one of the Soviet veterans who liberated Auschwitz. At the time of the liberation he was 21 years old and was commander of machine gun platoon. Here is translation of his recollection:

We did not know anything about concentration camps, all the way from Krakow and further one we had no idea of the existence of Auschwitz. Our task was to secure the front line. After advancing through the village we came across a huge field, which was fenced around by razor wire. The field had massive looking concrete pillars with electric razor wire, it had those electric rollers on them.

You see the ovens used did not fully destroy the bodies. As such, what was left was put in stacks on top of each other, put tree trunks on top and burn some more. The weather was nasty that day, everything surrounding the camp was covered in snow. Inside...everybody was black from soot, the ever present smell of burnt things, burning human flesh. It never lived you, it was an ever present smell.

We approached the wire. Face to face with the prisoners. Their faces black from soot. Some of them were covered with bed sheets, type of plaids. Their only bright things we could see were their eyes. They were shining, brilliant looking. The eyes are the only thing we could judge the mood they were in and they looked happy, jubilant even. We could not speak to them. We tried Russian, Polish to no avail. I think they shouted "HUNGARY! HUNGARY!". I didn't know at that time that it mean "Hungarian" in most of the European languages. The eyes...it was as if you could see that they walked through Hell, yet the looked thankful to us and happy to see us. Those eyes.

• Found another. Another veteran recollected the meeting. Approaching the camp we saw smoke coming out of the chimneys, razor wire, and standing skeletons covered in stripped clothes. They were all over us, shouting in different languages, hugging us, asking for food. Later, we saw human bones in the ovens, bags and bags filled with hair, human skin, and the houses where the guards lived: private swimming pools, gazebos..."

Sources: Here, here, and here.