I can’t find anything about it online, so I don’t know where they got it from.
Marx saw the peasantry as a conservative and maybe even a reactionary force. This was based on his study of french history, where peasants caused Napoleon III to get into power. In many countries, the peasantry was the largest class, not the proletariat. For the proletariat, the revolutionary class, to take power, it would need to be the largest. This of course could be misinterpreted as an absurd position that peasants need to be killed, when what was actually meant was that the country would need to industrialize far enough to create a sizable proletariat.
Either way, history shows that an alliance of the proletariat and peasantry is capable of revolution.
He thought the peasant class would be killed off, yes. As in, as industrialization would continue and capitalism disseminate, there would be no peasant class anymore.
The individual peasants would, in the process, either die of old age or become in essence either bourgeois, petit-bourgeois or, in the vast majority of cases, proles.
Maybe they misunderstood the description of the already underway creation of the proletariat as some sort of plan to force peasants off their land and into the cities?
Communists: Capitalism is going to make peasants move to the cities and become proletarian, eliminating the peasantry as a class.
Reactionaries: Hey peasants! The communists are talking about eliminating you.
Lol no way. He didn't think they had particular revolutionary potential, perhaps. But absolutely be didn't want them killed off lol