Does (bad) karma have to be depleted to be able to attain enlightenment?
While some of the past karma may fruit as obstacles to liberation, not all karma has to fruit before liberation, as even Buddha himself famously experienced results of his past karma after his enlightenment (e.g. when Devadatta wounded the Buddha's foot with a rock).

As I explained in https://buddhism.stackexchange.com/a/1672/43 Buddha rejected the theory of intentionally "burning" past karma through exertion and austerity. Instead, liberation is achieved by "abandoning of unskillful mental qualities and the attainment of skillful mental qualities in the here-&amp;-now".

Basically, Buddha's advice to people of difficult backgrounds has usually been to leave the past in the past, and focus on the choices to be made and the state of mind to be maintained from now on.