As an early employee you have a lot of credit with the founding team, if you aggregate the concerns of others and go talk with them that the team lead is overtly negative and weighing down team morale then they will probably listen to you.

Keeping, motivating and retaining key employees is a core task of a manager, if the founders of the company realize this then they should also see that they have to take corrective action against the team lead (which can range from a conversation to outright firing).

In the unlikely case that the founding team doesn't see your value then you're better off leaving but always be very open about the reason why you are leaving so they can change their mind.

Also keep in mind, that if the founders are assholes you might find yourself without a job for being very open. So you might want to line up another job before initiating this process.

edit: Probably having an open conversation with your team lead first is a good idea; even though it might not help anything at least you can tell the founders that you tried to talk with him