I would like to use the sentence
He was ‘over-eager’ to implement his plan, leading to failure.
I am looking for a single word which expresses over-eagerness in a negative sense.
Impetuous has a similar negative connotation. Oxford Dictionaries:
Acting or done quickly and without thought or care.
An impetuous person acts impulsively and does not think through their actions. Impetuousness would explain failure. Hence one could say the following:
He was too impetuous in implementing his plan, which led to its failure.