thers wouldn't be resentful of their harsh lives
Agreed. The ones that pop into my head are Angron and Kurze.
I believe nurture plays a bigger role in determining who a person is over their inherited genetics.
In real life, I agree. There does, however, seem to be some sort of genetic...predisposition. All the Legions (to my memory) were roughly in line with their geneseed from the get-go. Generally meeting up with the Primarch merely reinforced and refined their habits and skills.
This makes me believe that the Primarchs were genetically predisposed to certain traits.