Humanity isn't nice, and convictions/touchstones kinda need to be dark to work
First off, this is about mechanics, not about limits. Games are meant to be fun, and everyone has limits to what is fun. Obviously, work out with your group what are hard limits, and what is just good roleplay. Now, moving on to the actual points after the mandatory disclaimer:
Ok, so think about the game mechanics... You have Chronicle Tenets. These are the "It's sorta not ok to do this, but it's still allowed to roleplay this" actions that the player can take, but with consequences than range from 1 to 3 stains. Lets say a Chronicle has the following Tenets
>Petty theft up to 30 hours minimum wage = 1 stain
Major theft over 30 hours minimum wage = 2 stains
Blackmail, extorting, non-con intimacy (inc. feeding) = 1 stain
Human trafficking, slavery, addiction creation, blood bonding = 2 stains
Deliberate, unnecessary non-superficial injury = 1 stain
Deliberate, unnecessary permanent injury = 2 stains
Deliberate, unnecessary ending of mortal/kindred = 3 stains
Deliberate reckless endangerment (inc. Masquerade breaches and not staying fed) = 1 stain
Now, none of these things are what you'd expect from polite human behavior in your daily life, but these are all things humans actually do, which brings me to my first point: Humanity isn't a measure of politeness, niceness, nor morality, and neither is the inverse true. Consider the following moral issue with organ harvesting: Humans have two kidneys, two liver halves, two lungs, a heart and plentiful bone marrow, all of which can save a human life, which means more lives can be saved than lost by killing and harvesting humans for parts. This is a moral argument centered on the greater good, but it is something most would also consider inhuman to even consider. Moral does not equal human.
Further, unless you're a Farmer or Consentualist, you've already got some issues, but lets say you want to roleplay a Brujah or Gangrel street ganger who gets into violent fights at the drop of a hat, or a Domitor, or a Ventrue mob boss running a drug ring... that's a one way ticket to wighthood, mechanically, which really limits role-playing. So, how do you roleplay complex characters instead of My Little Pony: Friendship has Fangs Now.
This is where Convictions come in, allowing you to block 1-3 of the up to 3 Chronicle Tenet stains so long as you have a good role playing reason to do so, and an example of a Touchstone who can show that this is a HUMAN, not MORAL, life path... but at the cost of taking Stains if you turn you back on the Convictions keeping you human.
For example, if you wanted to play the drug running Ventrue mob boss, you might take the following:
>1) Business is Business: Take one less stain from harm caused by situations created to make money, but take a stain when deliberately getting in the way of making money.
Touchstone: a human embezzler who scammed countless working class elderly out of their retirements with a ponzi scheme.
>2) Power has its Price: Take one less stain from harm caused in order to gain or maintain power, station and control, but take a stain when deliberately risking your position or control.
Touchstone: a human politician who has screwed over countless people by letting corporations dump toxic waste in return for campaign favors.
>3) Not my Brother's Keeper: Take one less stain for creating situations where people freely harm themselves with your enabling, but take a stain when you try to restrict others from making their own bad choices.
Touchstone: a human meth dealer who sells to highschoolers in order to keep a stead market of addicts, uncaring of the lives of addiction and prostitution created.
These actually create mechanical benefits to prevent Stains from your role-playing choices while forcing you to confront the reality of the character you've chosen to play and what that means for the world. You CAN play this character as part of Humanity, but you have to confront the part of Humanity that represents. You can sell drugs because you're making money (1), and it's their choice to buy (3), you can feed on the unwilling since it lets you use powers and maintain control (2), and you can harm people who get in the way of you making money as a mob boss since it's profit and power lost (1 & 2), plus it was their choice to go after you (3)... But what does it make you to do that? You have to think about your convictions to apply them, and consider who else that resembles... and if that's who you really want to be!
Now, consider the more vanilla alternative:
>1) It's Nice to be Nice: Take one less stain from being nice to people, but take a stain whenever you're a dick to others.
Touchstone: Your sweet grandma who never hurt a soul and who bakes cookies for the homeless with her church group
See the problem? You didn't fix the feeding issue, or the dangers of violence eroding who you are... You just added another burden, because now every time you blow a fuse or flip off traffic, memories of how you failed to live up to your sweet ol' grandma's example will remind you of how far you've fallen from her version of humanity, and drag you down into giving up and becoming a mindless beast in eternal wighthood.
Humanity isn't about being moral or nice, and Convictions and Touchstones exist to remind of that there is Humanity even in the worst atrocities and foulest examples of human kind.