Option c is the most possible one out of all the options you gave. Also The same thing happened in the good doctor too! I guess this is how it usually works in kdramas.
Ah, what an ending. Great script, all plot points came together nicely. The most insignificant, mostly neglected,…
I think he had a mental issue. The traumatic experience he had at such a young age left a scar on his mind. So He assumed that he is bound to do the same because it's in his genes. Psychopaths don't feel guilt but Remember he said he regretted killing. So i think there is a high chance that his life would have been completely different had his mother not died or the detective hadn't abandoned him.
4 
7 
1 
So i think there is a high chance that his life would have been completely different had his mother not died or the detective hadn't abandoned him.
She must have asked for his help when her colleague committed suicide.