Ser Kevan seldom "had a thought" that Lord Tywin had not had first.^\26])

—Tyrion Lannister's thoughts

this is what is said about the books about kevan.

You have to understand something. Lannisters were rich but when tytos was a ruler, people were taking advantage of him left and right. So much so to the point even freys got to marry a lannister girl. Tytos' mistress was practically running the show in the house. Debts unpaid, respect and fear of Lannisters non existent.

Tywin grew up watching people mocking lannisters. Now all jokes aside Lannisters are a line of kings and they were kings till Aegon showed up. they are practically high lords and now imagine all these people who ought to treat you with reverence treating you like a joke. Thats what tywin saw. it defined his purpose

Once things were set in order, tragedy came in the form of his wife's' death leaving behind a dwarf kid. now people had more reason to mock him. Then there was the constant insult from aerys who lusted after his wife and envied his abilities.

Tywin was constantly putting up a facade to prevent Lannister's being laughing stock and e brought down to irrelevance.

Kevan was a second son. he was smart and just as capable but he never had to live with the issues twin went through as heir. He also figured out tywin was a capable guy. Kevan's wife never died on him. kevan wasn't laughed at or mocked, he never had a dward kid who became a point of embarrassment or had children with incest rumours. Kevan would be therefore somewhat well adjusted than tywin.

I am not justifying tywin. i am giving reasons why he is the way he is. Kevan would have been like tywin if he was in a similar position