Biography
There's a reason why it's Kick the Dog and not Hit the Dog.note While Violence Is Disturbing, there's something about a character using their feet to hurt someone that makes viewers especially uncomfortable. Attacking someone on the ground (or in a similar helpless position) is already seen as uncalled-fornote , but this moral disgust seems to be magnified if the person uses their feet to do it.
Tropes like Hand Stomp, Kick Them While They Are Down, and Finishing Stomp are almost always acted upon by villains, Anti-Heroes and Nominal Heroes. If a hero does something like this, they can very easily become Unintentionally Unsympathetic or a Designated Hero. Punching someone who's on the ground is sometimes okay, but stomping someone's face or throat is usually not. Oftentimes, other characters will react with shock and even horror to this.
There are three main aspects of this:
Power Imbalance: For one thing, feet are widely viewed as symbols of power in most cultures, but not so much creative power like hands are. Hands are associated with control, strength, and manipulation, but are also associated with art, expression, gentleness, peace, and so on. Feet are almost exclusively seen as symbols of authority and destruction, as well as submission to power.note
Brutality: Another aspect of this is the obvious fact that it's a lot easier to physically hurt someone with your feet than other unarmed methods, especially if the victim is on the ground. Depending on the size of the person (and what kind of shoes they're wearing), engaging in this trope in Real Life can easily lead to death.
Humiliation: And of course, your feet are on the ground almost all the time, giving the implication that the act of cruelty is also an act of humiliation, making the motivation similar to Dominance Through Furniture. There is the implication that their victim is not worth their time, as if they are nothing but a bug to them. Oftentimes, this trope accentuates a villain's pettiness and disdain when they do this, as well as their cruelty.note
Naturally, this can be considered a Sister Trope of Kick Them While They Are Down, but the difference is that the act of cruelty has to literally be committed by the character's feet. The power imbalance is key here. Kicks being used in a fight scene is not an example unless one of the characters is on the ground or otherwise helpless. Compare and contrast Finishing Stomp, Curb-Stomp Battle, Dismissive Kick, Kick the Dog, Trampled Underfoot, Weapon Stomp, Agitated Item Stomping, Crush the Keepsake, and Hand Stomp, which can all be considered Sub-Tropes of this. Also compare Giant Foot of Stomping and I Kiss Your Foot, which can be an example of this if the intention is to humiliate or bully someone. Also see Dominance Through Furniture, which is similar thematically. Not to be confused with Agony of the Feet, which is when one's own foot gets hurt from stepping on something.
As this is a trope related to morality, No Real Life Examples, Please!