Employees make mistakes and to punish them for it is insane. If you were purposely trying to screw things up and were fired for that, of course thats a different story- but it sounds like you were just trying to do your job and made a mistake, it happens. Also, seeing as you were supposed to have been paid the bonus while you were there, it makes it twice as awkward.
This should be a lesson to other founders- Treat people good and they will speak highly of you and may stay with you forever, to your next startup or company. Dont and they will do the opposite. That alone is worth more than the 10k you save.
Penny wise but pound foolish, as the saying goes.