The serum amplifies everything that is inside, so good becomes great.
Bad becomes worse. This is why you were chosen. Because a strong man who
has known power all his life may lose respect for that power, but a
weak man knows the value of strenght. And knows compassion.
Dr. Bruce Banner murdering the avengers without even turning into Hulk.
Listen, Tony might snark at you but he cries himself to sleep daily. Meanwhile, Bruce will dismantle you with words and make you question your worth at 3 am in the morning.