its quite simple actually,kratos was blinded by his rage ,in god of war 1 and chains of olympus and such u can clearly see him still being human and having parts of it deep down,in chains he was a delusional man still believing in the gods of olympus and atlus talks to him about this sad belief in them cuz its all he has really,in 1 hes a broken man just wanting to end it all,ghost of sparta makes this change in his character from 1 to 2 make even more sense cuz it shows us a journey of him losing even more faith in them and by the end of that game he says they will pay,and in 2 hes so insanely blinded that he rages wars and he even takes the blade of olympus from zeus even tho his trust is very low just because he wants to fufill what he wants THAT bad,hes such an angry person in 2 that everyone he beats or is done with ends up dyin brutally weither burning even tho they helped him or like Icarus dying one of the most pathetic deaths,like look at every boss and u will see hes so insanely brutal with his ways of killing them,throwing them from windows to sharp things,ramming doors against their heads,etc,a man pushed so much and blinded by his revenge and rage would of course do this. in 1 he killed cuz it was his job and he killed in a fast and normal way as best as he can with his situation,even the hydra was cuz he needed to kill the heads as best he could with what he had,in 2 u can tell hes not just killing normally,he puts so much of his emotions into it,he even slams the translators's heads against the books and acts so angry to them and everyone all the time,in 2 he was just a crazy pyschopathic monster.oh also,a guy like him that has went this far and has lost so much including that spartan soldier in 2,not to mention his brother and mother and wife and daughter (his daughter twice) would start thinking of fixing it? probably not ,to him he was so far and when he had a vision of his wife in 2 he was so sad and asked her to forgive him,i think thats another reason,even if u ignore the rage i dont think kratos was gonna think he was worthy of having his old life back after all of the horrors he did ,even in the most recent games kratos is still struggling to truly forgive himself and move on entirely