Niggling thought of the day

This has been bothering me ever since "The Poison Sky", but... how is it that, when a bad or morally ambiguous character on a TV programme or in a film commits an act of self-sacrifice after recognising that they've done wrong, why is it that people describe this act as "redemptive"? As opposed to if, say, the character recognised the evil of their ways, but then went on with the rest of their lives, living with the shame of the past and doing their best to make up for it, and make others' lives better in the process? Taking a bullet for the Doctor, or whatever, may make for a nice exciting climax and a slightly poignant episode ending, but, in real terms, it's taking the easy way out, and neatly avoiding any uncomfortable questions about forgiving your enemies.