What is the best martial art to learn if you want to seriously injure your opponent?

It is called 'Thugery" and is all too common.
Happily, martial artists defeat them easily.

Anybody who would answer your question by listing techniques or methods for you to seriously injure somebody is highly irresponsible. However, that being said, any martial art can be abused - it depends on the attitude of the practitioner.

To seriously injure someone is a state of mind not a martial arts style. If you want to learn self defense you must therefore train for self defense rather than sport with the emphasis on scoring points and being limited by rules that protect the opponent. So find a teacher who teaches self defense and not sport. The techniques in a style can be used many different ways and a teacher determines which way.

Join the police academy

Best Martial Art To Learn

Practice Karate at Cobra Kai and remember to sweep the leg.

Pencak Silat or Din Mak

Eggsma-shin

The art of getting a gun and shooting someone in the face.

mcmap is the only martial art i know of that is all about battle and killing and ****. i suppose i know of a plethora of others that are like that too but they were all made back in ancient days with swords n ****. but styles that aim to kill include a whole lot of other martial arts, the unique stuff is probably only the rifle stuff where your always trying to get back to your weapon and using the weapon in close quarters to stab and ****. probably all of the other stuff is taken from other martial arts to make it specialised for combat. but usually when your trying to seriously injure or kill someone, having close quarters scuffle almost never happens, you either shoot them from range or stab them when they arent looking. i cant think of any other martial art thats designed to actually injure or kill, that isnt ancient. other modern arts are mostly based around incapacitating someone in a hand2hand fight or a knife fight like kali or something. good luck trying to learn something like that cause you have to enlist to learn stuff like that.