Ironically, probably the one thing Sebastian learned the most from Demetrius is how to use words to hurt people/keep them away from himself.
They don’t really have healthy discussions anymore. They have periods of tense ‘civility’ until the ticking time bomb explodes into another fight and the cycle resets itself again and again. And those ‘civil’ periods are full of passive aggressive and increasingly cold and venomous jabs aimed at each other. Poking at the holes they’ve memorized in each other’s armor. Neither willing to bow down and ‘play nice’ even when trying to give the impression of getting along.
That two heart event with Maru is the kind of shit that Sebastian lives with on a near daily basis, and he’s learned how to turn it right back on him. He sees through Demetrius’s bullshit bc he isn’t blinded by parental love like Maru may very well be.
He sees his stepdad for exactly who he is. And as the only father he’s ever known, it’s inevitable that he’d take from his environment what he could.
Where Robin’s temper is like fire, quick and burning bright when it’s lit, Sebastian took to Demetrius’s ICE instead. Not completely; his own temper can ignite as his last defense against someone getting too close or the need for a physical altercation arises, and he most definitely gets in screaming matches the most easily with Demetrius, but he’s otherwise almost chillingly calm when verbally cutting someone down. Complete with a glare that’s as cold as winter itself. With words that can drip with his stepdad’s poison if and when he needs them to.
Sebastian has more in common with him, or even just more he can tribute to Demetrius’s influence, than he’d ever care to admit.