It's probably a good thing that he's just a fool.
A full-fledged cold-hearted young filthy-rich villain would not have any of this talk with our Niihama-kun and would instead quietly hire a hitman.
A psychotic agitated young filthy-rich villain (worthy of some “
Male Yandere” tag) would instead be 100% sure that a death is not really much of a punishment for “a crime of going near Haruka” and thus would intend to introduce an element of years-long suffering by doing something even more cruel and sinister that would have an immediate, and permanent, and devastating effect on Niihama-kun; for example, would hire some thugs to attack his immediate family with the intention of killing Niihama-kun's mother and simultaneously leaving Niihama-kun's
imouto badly injured and crippled for life so that she cannot go through many minute details of her life without Niihama-kun's careful assistance, so that Niihama-kun cannot really spare any time for Shijouin-san and their relationship is strained and then broken.
That could be an interesting alternative explanation behind “this is the first time Haruka went home with a boy” (what if Mutsurugi just could get rid of any such boy earlier, acting like one of the antagonists in Prosper Mérimée's “Chronique du règne de Charles IX”), but the author is unlikely to explore this option, and that's a good thing.