1) he took the serum after the rude Wakandan ladies that weren't even supersoldiers waaahhhh kicked his easily bruised ego. But he's been wanting it since day one, since he seems to believe that's what it takes to be Captain America, and not, you know, doing it for the right reasons, being compassionate, and not giving a fuck about impressing your superiors.
2) that was 100% him. Serum hasn't turned any of them into ragemonsters, they seem completely in control of their faculties.