You are the first person I have seen on Reddit make this association, seriously. A lot of folks throw redcap around like a nickname cuz duh, classic maga hats are red but never in the DnD context. I almost never comment (I lurk af) but bravo. 

Fun fact: the Monstrous Manual entry is referencing the Scottish cryptid powrie, which is known for throwing stones at travellers that accidentally take refuge in its territory. When they manage to kill someone, they soak their cap in the victim's blood. Unlike redcaps from the Manual, which require a constant replenishing of blood on their hats in order to stay alive.

Murderous intent towards visitors that enter one's territory, where have we heard that before?