I also left my startup due to ethical concerns. First, keep in mind that if they are this disorganized and vengeful, people probably already know and will sympathize with you. Second, if you're not a founder and didn't work there long, you can leave it off your resume. Third, if they do badmouth a fresh employee, I think that's a reflection more on them than you. Hearing that you left a startup quickly would mostly make me wonder why rather than assume anything about you.
I know it can be scary especially if they lie about you behind your back. However, people gossip unfoundedly all the time, and there isn't much you can do about it except continue to be a good person. As other commenters mentioned, the tech world is large and the vast majority of people you talk to will never hear or care about the gossip.