It's dog-eat-dog, from humans on down to the microscopic bacteria that feast upon living organisms.
Your past self was not cruel, but rather, ignorant and uneducated. Our past selves either received the guidance we needed or we didn't. Some of us didn't need the guidance because some people are just innately wired to compete and succeed while others are not.
Yes, your past life is responsible for where you are now: we are the sum total of our choices, but you have to factor in environment as well.
Some of us, are in an environment that fosters success, while others are taught to scrape by and barely survive. If your parents are negligent, it is 1000x harder for you to find a good mentor and rise above. People do it, but they are the exception rather than the rule.