Do you really think the "original understanding" extended to allowing violent felons to be allowed to have guns. I wouldn't think so.
No, nor did the Heller Court. Or more precisely, it never commented on the issue, but stated its decision should not be inferred by lower courts to authorize striking down laws prohibiting felons from owning firearms.
when a person pays a debt do you continue charging them for a debt that is already paid?
When they are released from prison their debt is paid.
As we know no right is absolute – including the right to self-defense or to own a handgun. The right to free speech doesn’t extent to camping out in National parks, for example.
The state may preempt or restrict a right if it can demonstrate a compelling governmental interest and substantial evidence in support. It can be argued that keeping firearms out of the hands of convicted felons is a legitimate state interest, and would likely withstand challenge.