What you have is a clash of expectations. Your group likes to plan and strategize, she clearly doesn't (or rather, planning and strategizing is less important to her than playing this impulsive character). It's really important to articulate the expectations of play and you should do talk to her, and the group, directly, rather than create an NPC to try to block her character.
You'll notice if you look at other media or stories, there's a big difference between impulsive characters who TALK about jumping in, but don't actually do so, vs. those who DO and cause a lot of chaos.
If your group expectations are that everyone is mostly working together and only conflicting/at odds for show ("The elf and the dwarf are arguing again." "Oh, let them be, they're actually good friends.") and she is operating on the idea that each character might have very different agendas or less concern for the group, you're going to have clashes.
Being clear about what kind of game this is, helps her decide if this is the game she wants to play or not.
I'd suggest you put yourself in their shoes. For a better understanding of how they feel, imagine the game as a movie with their character as the hero.
In a movie, if the hero heroically struggles with evil and some bystander steps up and kills it when it was almost defeated by the hero, would you think that's a fun scene? Would you want more scenes like that in the movie?
Chances are, your answer is "no". Their answer seems to be "no". The relative rarity of movies that work this way implies a majority of people does not like this, even though mechanically, the audience does not get anything from the scene, no XP, no money, they just take away how it felt. And it did not feel good.
I think they may feel cheated?
They probably do. They put in effort and risk and it seems the reward for it is claimed by others that did not put in either. The reward might well be that feeling, not loot or XP.
In the three scenes you described, was there any reason for them not to be the hero? Why did the NPC step up all of a sudden when the plan was for the fighter to hold and the others to go for another goal? Why did the PC "walk up" leisurely to deliver the killing blow in the middle of a chaotic fight? Why the hellish rebuke trap? What was it good for, other than to steal his heroic ending of the scene?
In real teamwork, the others share the risk. All fight and by random chance one will get the killing blow. If all share the risk, then it's teamwork. If only they are at risk, then they should get the good feeling of having overcome it.
As a player, I'm not really sure how this should be handled.
66% of your mentioned examples are DM made, so I'm not sure if this is something that can be handled by a player alone. Talk to the group as a whole, including the GM. Maybe do the same exercise, let them imagine the scenes as a movie and ask them if they would want to watch it.
Best Answer
You need to talk to everybody
Clearly, you're no longer enjoying the campaign as it is. You came into the game with a different expectation than what you're getting, and that's what a Session 0 is for.
Point out that you thought that this was going to be a game about adventuring heroes and that not everybody is in agreement on that idea.
You're traveling with a known murderer, are aware of that fact, and haven't made any attempts to arrest him for his crimes. In any realistic world, you're the guy's accomplices.