In capitalism there is every incentive to be cruel to other people. Most people probably feel horrible when seeing a homeless person on the street. We want to help them but we are told that helping them is bad, because they want to be homeless and if we give money they’ll just ‘waste it’. That’s not a normal human reaction, that’s learned behaviour.
Those specific cases are learned behaviour, but general purpose cruelty and theft is something that's been around longer than capitalism has, and has to be unlearned as kids, not learned.
It's true that cruelty has been around for longer than capitalism; it's untrue, though, that it has to be "unlearned". Every experiment (that I've learned about) demonstrated that even toddlers care about fairness, which (I would argue) is inherently opposed to cruelty