Is it a bad heavy karma to criticize or give up your guru?
Heavy bad kamma is always a matter of strong wrong view, and ingratitude is such especially if it's toward real and lasting help: Toward one's Gods, Parents, people of goodness.

Much done.

It's how ever not bad if based on right view and if the relation is a more or lesser good trade with the world, it would be even good to seek for leaving, correct of course and not just for another similar way.

[Not given for trade, exchange, stacks and what ever relations not dedicated toward real liberation]