A man crushes another man's skull while he's screaming for help. We can see his face getting crushed.
Season 5 contains more graphic violence than the previous seasons. Includes throats sliced, blood splattering, severed limbs, characters crushed to death, etc. One character's heart is ripped out of his chest.
Violent content increases as the series progresses: first seasons contain intense sci-fi type action and some deaths. Later seasons have more strong violence and scenes of torture, plus blood and some gore present. S4 to S6 are the most violent from my perception. S7 contains an EP that shows a lot of gore but it's mostly exaggerated.
There is a lot of torture. Graphic torture and screaming
After season 3, violence gets pretty bloody and graphic.
In season 8, there is almost no blood or strong violence up to episode 6. In episode 7 it gets really strong, with people being drilled, sawed, and brutally mauled. This contains very high amounts of blood like what you'd find in an R-rated movie.
During an autopsy scene, a man has the top of his head cut, revealing the skull. The surgeon then picks into and removes it, showing the dead man's brain.
During season 4, there are hyper-realistic robots that look like humans that attack the agents. Due to this, there is intense action with breaking their heads, power-drilling into their eyes, and dismemberment. There is blood in the robots to add realism to them. This makes for more violent scenes.
There is frequent hand-to-hand combat in the series, as well as gunfights and brutal action violence. Injuries are usually bloody, and characters beaten or bruised, but although the action is intense at times the main characters are generally good guys who win in the end. However, villains often die as well as side characters.
We see a severed hand on the floor of a train. Slightly graphic.
A woman is burnt to a crisp by a man with pyrokinetic powers. We see her body turn completely to ash.
A man tears through another man's chest and pulls out a rib, which is seen clearly. Both the rib and his hand are bloody.
A young man freezes another man with his cryokinetic powers and his body breaks when it is pushed to the ground.
A woman electrocutes another woman's face with a wire.
A man with alien markings all over his body uses a knife to make some cuts on another man's body and he screams.
A woman is vivisected by a HYDRA leader to obtain her regeneration powers to make himself younger. We see a sequence of disturbing images of her skin being sliced open during the operation and also see her insides, with some organs being removed.
Many random henchmen characters are shot down throughout the series.