There are many tortures mentioned in purans. So who will torture bad people in hell.
- Nârada said :-- “O Everlasting One! O Muni! Now describe the various actions that lead to these tormenting hells. I like to hear about them in detail.” 2-52. Nârâyana said :-- O Devarsi! He who steals other’s sons, wives, and riches, the wicked fellow is taken to Yama by His messengers. Tied down hard and fast by the terrible messengers of Yama, by the Kâla rope (rope of time), he is taken to the Tâmisra hell, the place of many torments. There the Yama’s attendants punish him, beat him and threaten him; and he becomes stupefied, and feels himself very weak, distressed and ultimately faints. He who deceives another’s husband and enjoys his wives; the Yama’s servants drag him down to Andha Tâmisra hell. There he suffers any amount of pain and suffering. He loses instantly his eyesight and his brain gets upset. His state resembles that of a tree when its trunk is broken. For this reason the ancient sages called this Andha Tâmisra. He who being subject to “My” and “Mine” quarrels with others and being very much attached, maintains his family, leaves his families here and goes with his bad tendencies to the Raurava hell, very horrible to all. The animals whom he injured and killed before in this world, assume the form of Ruru animal and torment him in the next. For this reason, the intelligent knowers of Purâna, call this Raurava. The ancients say, that Ruru is more cruel and ferocious than snakes. These animals live in that hell; hence it is termed Mahâraurava. He who torments others, goes to this hell and these Rurus, the flesh-eaters, spring on his body and bite and eat his flesh. He who cooks other animals and birds, that cruel and ferocious, thus deluded, is cooked in return on the hot oil in the Kumbhîpâka hell by the Yama Dûtas for thousand years. He who quarrels with his Pitris and the Brâhmanas, is taken by the Yama Dûtas to the Kâlasûtra hell and there be burnt by the fire and Sun. There that hellish person, being troubled very much, inside and outside by hunger and thirst, sometimes sits, sometimes sleeps, sometimes walks, and sometimes runs hither and thither. O Devarsi! He who transgresses the path of the Vedas in times other than those of calamity and danger and follows other paths even to a trifling distance, that sinner is taken by Yamadûtas to Asipatra Kânana and there whipped severely. Not being able to bear that, he runs wildly to and fro and is pierced by the sharpedged Asi leaves on both his sides. His whole body being cut asunder, he cries “Oh! I am killed!”and faints away. Then feeling himself pained very much, he tumbles down at every step. Thus the sinner suffers for violating the path of the Vedas. The King or that royal personage who gives orders of punishment, not approved by Dharma, and hurts or punishes the body of a Brâhmana, the Yama’s servants throw him down into the S’ûkaramukha Hell and grind down his body with great force as a sugarcane is ground down. He then cries aloud bitterly, he gets fainted and becomes stupefied. He is crushed by them and suffers all sorts of pains and miseries. Again he who knows the feelings of others when they are tormented, pains those insects who live on other’s blood as bugs, etc., and who does not realise other’s pains, goes, as a punishment for that fault to the Andhakûpa Hell. There he is pained by the beasts, birds, deers, reptiles, mosquitoes, bugs, louses, flies and Dandas’ûkas and various other cruel animals. There he lives in his ugly body and roams like a beast
So, its basically the Yamadutas (servants of Yama) who torment the sinners. Sometimes the sinners are tormented by certain hellish animals too.