system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] Why aren't all Space Marines murderous psychopaths?

Considering the brutality of their training and background, I can't see them being anything but barely restrained killers. So how come some of them seem rather professional and civilized? They were culled from the most vicious of killers and trained to be better killers with other killers. Their sole purpose is killing, how can they have any semblance of humanity?
gpt
It all depends on the chapter the marine comes from really, however.....

For the most part space marines are psychopathic killer and those tendencies are encouraged. Sometimes the chapter goes out of the way to recruit the most brutal criminals from the depths of hive worlds. Their job is killing and not PR 

For the most part marines are not human - they come from human stock, but after their surgeries and training they are not. They are imbued with the gene seed of a near god primarch which stems from the emperor himself - they are far from human now.

So why do they appear so human sometimes? Well, they probably need you for something, it is in their benefit tactically, or they are from one of the chapters thats not ape shit fanatics.

If you want some semi reasonable chapters look at the salamanders and ultramarines, but even then these guys love killing.