“Don’t think of it as killing. Don’t think at all.”
From the outset, the Vampire games (and most of the World Of Darkness games) have to some extent been about what it means to be human, about what you’d do with dangerous levels of power or an irresistible urge to harm others or the isolation of being unable to touch anything or... Vampires, in particular, struggle with the need for blood, the risk of Frenzy from hunger or rage, and the necessary brutalities of keeping the Masquerade.
Humanity is a pretty simple system for this, really. Check what a character just did against the Hierarchy Of Sins and roll some dice to see if they lose a point when they break the rules. You lose dice towards the end, but it actually gets a bit easier not to sin as it goes on, unlike the death spirals of injury (or the Sanity system in Call Of Cthulhu, where the same things trigger rolls at all points and you roll the dwindling stat itself) so you can bottom out around Humanity 4 and act much like a typical Kill Them And Take Their Stuff player character with few worries.
Even then, in the first edition of Masquerade, Humanity 0 characters could stay rational even if they were no longer officially playable - losing socialisation to the Beast came in later. Indeed, Chicago By Night 1 has some perfectly affable Humanity 0 characters. (And quite a few Humanity 9 and 10 ones for that matter, some of whom are saintly but some don’t seem all that pleasant...)
The Paths Of Enlightenment that came in with the playable Sabbat don’t change much in rule terms, just the specific codes to break (and whether you can resist or just ride the wave of frenzy). Now you’re violating honour, or self-involvement, or whatever, instead of being humane.
Note that I say humane, not human.
Humanity in Masquerade, and the first edition of Requiem, is at its core about caring for people, not trying to act like you still are one. The Humanity rules for the new Requiem rulebook Blood And Smoke: The Strix Chronicle, road tested in The Danse Macabre and previewed in the FREE adventure Reap The Whirlwind, look at that other definition.
It brings in a number of representative break points for every dot of Humanity, tying some to crime or violence and others to different facets of not being human any more, like frequency of significant human contact, and experiencing things humans could not and other reminders of being a vampire. The risk of Detachment might come from seeing a sibling die of old age as much as killing someone for their blood.
Or possibly your sire, despite you being a thousand years old.
In another big change you can buy off specific triggers (by taking balancing psychological or supernatural flaws called Banes). So you can shape the system somewhat to avoid rolling every time you deal with something that comes up regularly in your game. A vampire could easily become jaded about the necessary violence of feeding or even about killing for blood but still be disturbed by the passage of time. This lends the game a different feel than the fear of wrongdoing that the original Humanity system can generate.
Both versions are about how to be human, and encourage your character to engage with people and avoid the easy violent solutions.