The Cambodian genocide came to an end after the Vietnamese victory following invasion. But was the genocide one of the main reasons for invasion, or was it simply because Pol Pot was practicing the wrong "brand" of communism? Cambodians were fleeing the Khmer Rouge into Vietnam, so surely they must have known the massacres were taking place.
Hi, I've answered very similar questions like Why did Vietnam invade Cambodia to topple Pol Pot? What made Hanoi suddenly care about human rights violations? and When Vietnam invaded Cambodia and overthrew Pol Pot, where they aware of the level of atrocities he was carrying out and did they have any humanitarian motivations?
I think those should both answer your question, but a short summary would be something along the lines of... well, the way you put it 'because they were genuinely horrified by the genocide', well yeah I don't think that was the main thrust of it. To dig slightly deeper into the nuance however, aside from the issues of generally using the phrase "genocide" to describe what the Khmer Rouge were perpetrating against the majority of the population... (see this answer for more of an explanation of that) the Communist Party of Kampuchea were inflicting quite horrible death tolls against Vietnamese on the border regions, as well as to the remaining Vietnamese within the country. Their stated aims, against this group in particular, certainly do fit the UN definition of genocide, and the surviving leaders have been convicted of this crime (against this group - not the majority of victims: the Cambodians themselves which falls under the umbrella of crimes against humanity).
The Vietnamese communists, since the 30's, had engaged in vague ideation of an Indochinese Federation of communist states, that they would naturally lead as the begetters of communism to the region and the most advanced country within it... this was largely stifled in the wake of the Geneva Accords in 1954, but circumstances arising from the Sino-Soviet split as well as the aforementioned border clashes between their former Cambodian comrades and their own new socialist state led the two to war, and the most expedient end to that war was total invasion and installing a Cambodian government friendly to their own. Which is exactly what they did.
As one of the answers I've provided states, while it certainly took the shape of a humanitarian intervention, it was not quite the intention, nor primary motivation.