Why was Limon so fateful?

He was from the neighborhood Escobar built for the poor. Escobar was like a god to him.

In real life (according to a post about "28 things wrong with Narcos" written by Pablo Escobar's son) Limon wasn't really a guy who was hired near the end. In reality he had worked for Pablo's brother for 20 years and was a driver who would drive the truck to La Cathedral (the self made prison Pablo was in). So by the time the "end" came Limon had already been working with the family for decades.

I got the feeling that Limon had low self-esteem and became super-loyal to Pablo because he wanted to feel important.

Do you mean faithful?