I'm looking for the word to describe for someone who's so insecure about being the best, that they would go to great length to sabotage other peoples chances just to be the best.
For example, Lisa Simpson is known for this because whenever someone is better than her at something (music, school etc.), she'll get jealous and try to sabotage them.
Another example is Jerry Smith from Rick and Morty, who was willing to risk a whole eco system just to pretend he's right (pluto being a planet).
The word I've heard over the years was pretentious (attempting to impress by affecting greater importance or merit than is actually possessed) but I discard it because I didn't see the sabotage others in it.
Best Answer
While it doesn't have the connotation that the person was in the top spot like your examples,
Ultimately, the underminer is attempting to come out on top with their nefarious means, so it mostly works. It just lacks the connotation that the person has started there and is doing this just to keep their position. If that was just a side effect of the examples that came to mind, then there you go.