In most countries, there are specific laws regarding defending oneself and going too far.
If a thug in the street tries to strike you and you retaliate by striking him back then running away while he is dazed that's one thing, but if that strike knocked him out, then you continue to hit him/her whilst they are on the ground unable to defend themselves, then that would make you the aggressor and you would probably get into trouble for it.
To some people the above is obvious, but to many new martial artists who are not familiar with there countries laws, it is always best to research them so as to know how far to go on the street without getting yourself into trouble.
Up until a few years ago in many countries if you held a black belt in a specific martial art, it was your duty to inform the police so they could register your hands as so called "lethal weapons".
Although this is not the norm now, some authorities still see martial artists as people with an edge over untrained people and when they are involved in any kind of street fight, defending themselves or not, the fact that they are martial artists is taken into account and if they have been seen as using unnecessary force during that fight, they could get into a lot trouble for it.
MMA is my favourite sport, I love MMA training and think it is the number one fighting sport in the world with some of the most well trained athletes, but the fact is when some people see kicks against someone on the floor or ground and pound in MMA, they may get the impression that in the street this is what you must aim to do.
The aim is to defend yourself successfully.
If there is no need to keep hitting someone on the floor, DON'T DO IT.
If the person has a weapon and is intent on really hurting you, it may be necessary to use this kind of force.
You must correctly judge yourself, how far to go.
Defend yourself, but keep within the law.