system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] What chapter is most likely to fall to Chaos?

So, I was idly wondering *and definitely not planning anything nefarious*, what Astartes chapter is most likely to fall to Chaos? How you you get them to turn, and what God would they serve? *Definitely not doing that of course*, just trying to figure out how a traitor would come at them. Obviously. Why would you think anything different?

Conversely, and totally innocently ^I ^swear, what chapter would be the hardest to turn? How would you turn them? 

Not that anybody would, that would be crazy!

Me? I'm... uh... an Inquisitor? Yeah, an inquisitor. That's why I was thinking about it. Definitely thinking about how to spot the warning signs of chaos taint. Definitely not an Alpha infiltrator, and ^definitely ^not ^^trying ^^to ^^^win ^^^souls ^^^^for ^^^^Tzeentch. ^^^^^That ^^^^^would ^^^^^be ^^^^^crazy!
gpt
Any and all of them. The thing is that Space Marines are loyal to the emperor because they have been hypno-induced into loyalty. What i mean is that part of the "Space Marining" process of which gene-seed implantation is a part is the hypno-therapy which is used to implant the chapter's knowledge into the candidate's subconscious as well as responses very much akin to Pavlov's dogs. 

For example at the mere mention of Chaos a Marine despite never even seeing or hearing of it will bare his teeth  in automatic and extreme disgust. Upon mentioning some obscure name in the chapter's history the Marine will recall any and all pertinent information regarding said name. Upon hearing the word "Emperor" the Marine will experience extreme emotions of devotion and love. Upon reciting certain key phrases the Marine is able to initiate physical responses within their bodies such as increased adrenaline production or suppression should the situation call for it. 

Now why is this important? Its because indoctrination and hypno-therapy is actually extremely dangerous and reversible in extreme circumstances. In animals such as dogs things such as lightning can reverse the effects of hypno-therapy however in humans something more extreme is needed as a dog's tolerances are lower... in Space Marines the shock needed to strip away their hypnosis is very great indeed and is often the result of years of slow erosion or  facing Chaos for the first time. So the Marine who has been extremely devoted and stalwart in their love of the Emperor suddenly has his feelings stripped from him and he can see what he was i.e. a blind fool and will instead feel the other extreme i.e. hatred. 

This is why Chaos Marines despise the Emperor and feel contempt for Space Marines, its because they know how they were forced to love and how flimsy that love actually is. The depth of feeling has already been forcibly opened but now it is devoid of love, and the opposite of  love is hate... so they hate the Emperor with as deep and passionate of their previous love. So it doesnt actually matter what chapter (unless its the Grey Knights, their hypno-therapy was designed after the Great Crusade thus ensuring more avenues were covered to prevent shock reversal) it is, all Marines are susceptible to conversion. The easiest way to do it would be to convert the Apothecaries and have them fiddle with the Hypnosis machines and inserting a Key word which upon hearing it, the Marines will rise up and destroy their brethren from behind.