Kingsley is a ruthless business man who likes to use proxies and stand ins and all of them are expendable.
The employee of his that stumbled upon the green goblin cache and told him of it was soon dead after they took all the goblin gear and supplies to wear Kingsley could study them.
Kingsley saw a massive opportunity before him and chose to use it, but use it in his own way as Hobgoblin and not another Green Goblin.
Kingsley was also smart/wise enough to thoroughly study the goblin gear, get practiced with it, customize it to his own specifications, study the goblin journals and made sure he was ready and would not repeat the mistakes osborn made.
He braintwists another expendable proxie to recreate the formula knowing what would happen and used that bio data to perfect the formula for himself.
He made great use of having a twin brother to act as a doppelganger even sacrificing him against Phil/fake Hobgoblin.
He framed Flash as being the Hobgoblin and had Ned Leeds as a brain twisted agent to take the fatal fall to make everyone think the Hobgoblin was dead. I sincerely doubt that Leeds every really fought Spiderman as a Hobgoblin. he was just used for occasional stand in work and to take the fatal attack from those assassins so that Kingsley could disappear and retire.
He retires for years until Macendale disgraces the name Hobgoblin and returns to deal with him.
Nowadays Kingsley seems to be in the super villain franchise industry. Taking ID's and gear from super villains that are incarcerated or dead and franchising them out to new users for a tidy percentage of their take. If they don't pay, they get a visit from Kingsley.
Kingsley is narcissistic and a ruthless killer and that was BEFORE he got the goblin gear and became Hobgoblin. Back when he and SPidey first battled he was determined to prove he wasn't crazy like Osborn, and I think there was some question of whether or not the goblin serum did make him crazy or just enhanced his already existing ruthlessness.
I need to go back and reread those classics....
Anyway, framing Leeds and leaving him to die in his place is just part of Kingsley's ruthlessness. Leeds was a brain-twisted puppet to him, just like all the other henchmen that he disposed of both before and since.
Kingsley kills the henchmen that found and helped him gather the goblin gear.
Kingsley brain twists Lefty Donovan into recreating the goblin serum and used him as a test subject against Spider-man then killed him.
Kingsley brain twists Leeds and leaves him to die.
Kingsley sends his cowardly twin brother as the Hobgoblin to check things out and prepare for his imminent return to the city but stops his return when Phil as Fake Hobgoblin killed his brother.
Kingsley sends in another brain-twisted pawn into battle against Phil the Fake, the pawn is killed but Phil knew it wasn't the real Hobgoblin.
We see Kingsley back at his place in another country already brain twisting someone else to inevitably take the fall for him
Did we ever find out how Kingsley became so adept at mind-control? Was this in the Osborn journals (I don't think we've seen Osborn do this?), or is Kingsley just a talented hypnotist? That might be a good point of clarification if they ever decide to dig into his background.