I'm the only developer in a start up team and want to leave

1. Clean up the code as much as you can and document everything if you haven't already.

2. Explain your feelings and situation to the team, help them understand. Tell them you did the above and help them try to hire someone new.

3. Irregardless, leave if you are unhappy. No point in suffering longer if you're not happy. While doing the above 2, look for a replacement job.

Leave. You are not indispensible even if it feels like you are.

Tell the team, help them find your replacement.