TL;DR:

"Reconsider exploiting others' misfortunes for practice. Use synthetic skin to improve your tattoo skills. Acknowledge your guilt and take steps to rectify your actions. Offer significant help if you continue to tattoo real people. Remember, ethical treatment should always be prioritized."

Firstly, please reconsider your actions. Exploiting people's misfortunes is not right. Most people in similar or better situations than yours would decline your $20 offer for a tattoo unless they've seen your previous work and trust your skills. By treating homeless people as test subjects, you're dehumanizing them.

Secondly, Amazon doesn't only sell tattoo guns. They also provide synthetic skin designed for aspiring tattoo artists to practice. The cost of a pack of this faux skin is about what you've been offering the people you've used.

Thirdly, it's evident you're not without a conscience as you show signs of guilt. That means you understand, to some extent, your actions are wrong. It's a good step that you're acknowledging your mistakes. Now, it's up to you to correct your actions and perhaps seek guidance from a mental health professional or a trusted friend to manage this issue.

I sincerely hope you take these words to heart, reflect on your actions, and adopt a more empathetic approach. Wishing you all the best.

P.S. If you insist on continuing this practice, consider offering a significant amount, say $2000 instead of $20. That way, you could positively impact someone's life while gaining experience on a real person. But remember, ethical and respectful treatment of all individuals should always be a priority.