Okay, so, here's our theory. As we all know, Taka's backstory isn't even hinted because it's apparently too dark for a series that is LITERALLY about murder. The thing is, I think is IS hinted.
Let me explain.

1. Taka's family: So, as we all know, Taka's grandfather used to be Prime Minister but after doing some illegal actions he got kicked out of office, causing the Ishimaru family to fall into debt and for the Ishimaru name to be poisned. After this happened, my friend and I believe Taakaki became addicted to alcohol. Our proof for this? Well, Taka's father is a police officer, having a relative become a bad mark on the country would ABSOLUTELY affect how his work would go, along with the fact officers have a lot of stress and pressure put on them. So, I wouldn't be surprised if he fell into alcoholism. Also, JUST LOOK AT THE GUY! TAAKAKI LOOKS LIKE HE DIED AND THEN CAME BACK TO LIFE AND NO ONE TOLD HIM HE DIED.

So, our theory states that when Taakaki was drunk, which was often, he would take out all of his bad feelings on Taka. Beating him up and yelling horrible things at him. The thing is, in his drunken state, he confused Taka for a younger version of himself. I mean, Taka and his dad do look a lot alike. Would we be surprised if this was true? Anyhoo, when Taakaki wasn't drunk he was actually a very good a loving father and he would always feel horrible after doing such terrible things to his one and only son. Taka forgave him, of course, he knew that his father didn't mean it but he wish he could help his dad with his issues, but they never had enough money to send Taakaki to therapy. This, obviously, took a toll on Taka. He KNEW those words and actions weren't directed at him, but he couldn't help but feel like they were. 

Since all of these things pilled up on him, he took every failure hard, punishing himself in any way possible for being such a screw up. Wether that punishment come by his own hand or from someone else's. Evidence for this?

Okay, so, since we got that over, we can go to part two.

2. How this affected Taka: Well obviously, it made him wanna work hard so her could get a good job, send his dear father to therapy, and to restore the Ishimaru family name. He worked so hard on his academics he forgot about socializing. He became alienated from the other kids and his peers bullied him, and most likely used him for his academical talent. Therefore, causing Taka to grow up without ever having any true friends. Btw, all of the information for this part was gathered from Taka's FTE.

Anyways, I hoped you enjoyed this, and have a good day! And please, tell us what you think happened to Taka in the comments! (P.S. To add on to this theory, I have a headcannon that Taka's mom became a sex worker so she could make extra money for the family. She died when Taka was 6 from an STD she contracted from a costumer.)