If someone has made up their mind to do something evil, is there any benefit to them if someone else prevents them from carrying out their intended action?...I think you're right that often someone who has decided to do something wrong hasn't, for whatever reason, really contemplated all of the consequences of the action, and so is saved from something if prevented from committing the act decided on.
Virtue ethics (my usual framework) suggest that nothing much is achieved for the perpetrator. Once you’ve psyched yourself up to do a bad thing and overridden your qualms, the damage to your character has been done; carrying out the crime isn’t marginally worse for your soul. I think Catholic moral teaching might come to the same conclusion, since the moral actor has already given deliberate and complete consent to the act based on full knowledge of the gravity of the act.
But my philosophical intuitions don’t quite jibe with that conclusion. I suspect that a lot of the time, we end up surprised by the gravity of what we’ve done – that people rarely give manage deliberate and complete consent based on full knowledge....
I want to look at two additional ways in which behing stopped from committing a sin one had already decided to do would still have some moral benefit:
1) It seems to me that because we are not just a conscious will but also a body, that when we do something physically we commit to it and become attached to it in a way that just the mental decision to sin does not. In the Harry Potter example that's been discussed [spoiler warning]: Aside from any question of whether Draco really was mentally decided to kill Dumbledore, it seems to me that the physical act of killing the headmaster would have affected Draco in a deeper way than just the decision to do so. Although there's a culpability to decided to do an evil act, one isn't yet "someone who did that" with all the physical and mental sensations that come with that, until one actually does it. Similarly, say a married guy on a business trip asks a woman at a bar to come back to his hotel room with him, but then she turns him down or some practical circumstance prevents them from actually landing in bed together. Clearly, just by asking, he's betraying his wife in a very serious way. But it seems to me that actually completing the adultery is going to leave him much more attached to that sin, much more deeply in, than the unfulfilled decision.
2) In human experience, sin typically leads to more sin. People lie to cover up their transgressions. Hate breeds more hate. Violence leads to more violence, etc. Someone who's decided to commit some sin but is then stopped before carrying out the act may well not end up being drawn into the whole chain of related sins (lesser or greater) which would have followed in the wake of that first act.
Now clearly, the decision to sin is itself a sin. So it's not as if one is "saved from sinning" if physically prevented from carrying out the act that he or she has decided on. But I think that because we are both material and mental/spiritual creatures, being prevented from actually physically carrying out some sin decided on often does "save" us from something -- though it clearly doesn't render us innocent.
Now, where this would get very messy would be if the person committing the act actually thinks he has carried out the act. One of the things that makes Shakespeare's All's Well That Ends Well a morally ambiguous and in some ways unsatisfying play is that Count Bertram really does think that he was successfully committing adultery with the young Isabella when he was in fact sleeping with his wife. Even at the end of the play, when you get the big reveal that it was in fact his wife Helene he slept with and who is now carrying his child, the "happy ending" feels off because you get the sense that Bertram is still at heart disloyal to his wife. The "trick" didn't really work, in that while he was in fact sleeping with Helene, he thought it was Isabella and that is still who he really wants to be with, not his wife.