I regret becoming an autistic kid's guardian

Throwaway account for reasons.

Years ago I agreed to become the guardian of my half-sister's son. She is a single mom (dad is deceased). I knew he was autistic but I thought with today's knowledge of the condition and therapy things wouldn't be so bad. Fast forward to him now at age six and I'm growing to dislike the kid. The meltdowns, his mistreatment of animals, the constant supervision and care, you know how it is. I regret agreeing to be his caregiver if something were to happen to her. I feel so evil for wanting to back down, especially because I'm basically my sister's only option. The rest of our family is old.

I'm losing sleep because I keep thinking about if something were to happen to her, my life would change dramatically. I wouldn't even be able to have pets anymore because he abuses them. I think to myself that if things got really bad I'd put him in some special home, but I don't even know if my husband and I could afford that. We live in the U.S.

I don't know what to do. I feel so much guilt.

Edit: To clarify, I don't hate my nephew, but I'm realizing that I can't handle caring for him 24/7.

UPDATE: I'm getting more responses than I anticipated. I may not respond to everyone, but I'm reading/upvoting them all. Thank you everyone for your wisdom. I will be talking to my sister about my realizing that I won't be able to care for her son personally and about getting her life insurance to help my husband and I ensure that his needs are met for the rest of his life. I'll also be talking to her about what else we can explore to improve his behavior. He does go to behavioral therapy, but I'm learning from you guys that there's more that could be done. It's tricky because my sister is so stressed out, I don't want to come off as too pushy.