One thing that comes to mind when I think of Leo is cut-throat, only rivaled by Capricorn in this regard. From what I have seen, Leos are the quiet types that wait in the shadows when they are mad but when they deliver punishment, it lacks all empathy.
Looking back at a chart, I noticed that Pablo Escobar had a Pluto in Leo that touched a lot of his inner planets. Now obviously, while many modern astrologers claim that the sign is a softie, reality does not line up. The world saw what a fire heavy person in Pablo Escobar did to the crime world, I cannot think of a criminal more ruthless off the top of my head.
I feel like when Leos attack and kill, they do so not only lacking mercy but with such focus after a hurt pride that the target is wounded and destroyed beyond any means. It is no shock to me that so many Scorpios are fearful of the sign and have taken to bashing it all over the internet.
Maybe a part of why Scorpios hate Leo is also why they envy Capricorns, mars is exalted in Capricorn and Pluto is said to be exalted in Leo. Perhaps that talks about why the sign has such a hidden dark side that cannot be rivaled by any other sign except for Capricorn.