This is not something we intentionally do here, and is a feature of Stripe to automatically renew at the end of a trial if there is a payment method present. It should have also sent you an email about 7 days before it was going to renew.
With that said, I do understand how this may be unexpected. I will look into adding a workaround for this auto-renewal so that we can prevent that in the future for other users. Either way, if you contact support@kagi.com we can give you a full refund.