That would work for someone having an off day. Not an actual shitty person. If they knowingly didn't reply for half a year and only messages back when they wanted to use them, that's likely an actual shitty person.
Shitty people usually lack empathy, which is why they are ok with being shitty. If you treat them with kindness, they are much more likely to keep abusing the situation. When I refer to a shitty person, I'm talking about narcissistic, machevellian, slightly sociopathic types of people.
Realistically, if you have a large sum (millions) you can invest in safe blue chip stocks for steady dividend and long term ROI but if you're looking for short term gains, I would highly recommend you invest your money into your own business or education.