Genestealers are a type of Tyranid, that as their name implies, attempts to use their enemy's genetics against them. While they technically come in different types, they fulfill two roles: Pre-invasion Infiltration, and later on as shock troops and cannon fodder. They aren't specifically after Space Marine genetics though I'm sure they wouldn't upset if they did get themselves viable samples to integrate into the Tyranid biomass.

Pre-invasion, the infiltrate a populated area, with a combination of chemical agents and what can be surmounted as psionic abilities to obfuscate their clearly alien origin and hide from public view. They target weak minded individuals for brain washing, infecting them with a kind of virus that ensures their offspring are hybrid genestealers, growing the cult and gathering more followers to effectively worship the fullblooded genestealers who direct them in sabotaging the planet's defenses and preparing for the invasion. After a few generations, the cult starts producing more full blooded gene stealers, who spread out and restart the process until the invasion begins, with their followers being totally brainwashed and traitors to their origin,

So in short, you have a large group of people who were already fairly weak willed and probably uneducated, rallying around an alien creature who has brainwashed them into believing the actual loyalists and educated are the ones bowing to outsiders, actively trying to tear down their own government and make an alien invasion easier. Nothing about this is familiar at all. Nope.