I can't say I understand the first half of your ask... but, they mostly don't accept good things happening to others. they feel miserable because of all the anger/envy and other bad emotions they have bottled up so they lash on others & try to take that happiness away from them (cause they want others to be like them)

change is good (especially changing for the better) that person being forgiven totally depends on how much they're willing to change by understanding what they did wrong in the first place

but not all actions can be forgiven, and that's totally depends on the person and the damage they caused.. they may never be fully forgiven, but both them and the ones who were damaged by their actions can move on~