Indeed, while the books characterize Hagrid as a kind man who unintentionally endangers young schoolchildren because of his affinity for dangerous magical creatures, Hagrid might in fact be an evil spy, acting as a confidant to Voldemort for decades.
We learn that Hagrid was framed by young Tom Riddle for opening the Chamber, resulting in Hagrid's expulsion and subsequent keeping on as groundskeeper. Some 50-odd years later, Tom (now Voldemort) attacks Hogwarts and takes Hagrid captive.
Hagrid knew who Voldemort was before Voldemort appeared at the political scene of the magical world. He just refuses to use either of his names. And when he gave Harry the name, he gave him the name, Harry had to know. Hagrid very well knew, Voldemort was Tom Riddle.
At the beginning of the Battle of Hogwarts, the Death Eaters invaded the home of the Acromantula and drove them to attack the school. When the spiders attacked, Hagrid ran forwards to protect them. Acromantula are horribly strong, so it is likely that they captured him and took him to Lord Voldemort.
He is closely associated with the Order of the Phoenix, and has explicitly denied the anti-Muggle/Muggle-born ideology of the Death Eaters. There is quite literally NO plausible way Hagrid could be or ever have been a Death Eater.
In Hagrid's third year, he was framed by Tom Riddle for the crime of opening the Chamber of Secrets and using his pet Acromantula to attack several Muggle-born students and eventually killing one of them.
On confirming Draco's very much alive status, she plainly lied that Harry Potter was "dead". This action of hers was both very crucial and important, not only for Harry's survival but for Voldemort's final defeat as well, thus ensuring Harry was again protected by a mother's love, although indirectly.
At the beginning of the Battle of Hogwarts, the Death Eaters invaded the home of the Acromantula and drove them to attack the school. When the spiders attacked, Hagrid ran forwards to protect them. Acromantula are horribly strong, so it is likely that they captured him and took him to Lord Voldemort.
As a result, Hagrid appeared to others as the one who opened the Chamber of Secrets and had released the monster who lived within it. Therefore, Hagrid was expelled from Hogwarts and his wand was broken by others. This meant he wasn't allowed to complete his education and become a wizard.
Bellatrix Lestrange (née Black) is the first female Death Eater introduced in the books. Aunt of Draco Malfoy and Nymphadora Tonks. She was introduced in Harry Potter and the Order of the Phoenix. She is the most faithful member of Voldemort's inner circle.
On 31 July 1991, Rubeus Hagrid, despite being forbidden to perform magic, attempted to transfigure Dudley Dursley into a pig, after his father Vernon Dursley had crudely insulted Albus Dumbledore at the Hut-on-the-Rock, but only managed to give him a tail.
During the events of the second book, the Basilisk is unleashed once again and Hagrid is sent to the prison of Azkaban, as he is believed again to be responsible for the attacks.
She was killed in 1943 by the Serpent of Slytherin, which had been released by Tom Riddle. After the incident, she became a ghost who haunted the second-floor girls' bathroom (and occasionally other bathroom facilities) at Hogwarts.
The ones we know for certain that had overlapping Hogwarts years with Tom Riddle was Myrtle Warren (aka Moaning Myrtle), Rubeus Hagrid, and a group of Slytherins that became the first Death Eaters. The members of the group that were named were Avery, Rosier, Lestrange, Nott, Mulciber, and Dolohov.
Notably, Voldemort appeared to be entirely unaware of this split, as he later created a seventh Horcrux in his quest to have six, and continued to target Harry Potter despite him now holding part of Voldemort's own soul.
As revealed during his confrontation with Dumbledore, Draco was an insecure boy incapable of committing cold-blooded murder and was forced to do Voldemort's bidding under the threat of his and his parents' deaths.
As an adult, Lucius was an aristocratic wizard and head of the Malfoy family, believing strongly in notions of blood purity and the superiority of pure-blood wizards. He joined the Death Eaters, who shared his views on blood purity, and participated in the First Wizarding War.
In fact, Lucius' own support to Voldemort is revealed to come only under some form of duress, which is likely Narcissa's Veela Allure. When Lucius turns on Voldemort due to the reveal of his Horcruxes, Narcissa divorces him, taking 75% of Malfoy's wealth with her.
During the final battle, Bellatrix was the last Death Eater standing. She was eventually killed in a duel by Molly Weasley after her attempted murder of Ginny Weasley. Before her death, Bellatrix secretly gave birth to an illegitimate daughter named Delphini, whom she conceived with her beloved master, Lord Voldemort.
Something that people get wrong about Narcissa Malfoy is that she was a Death Eater, when in fact she was never part of Voldemort's inner circle of believers. She certainly affiliated with the Dark Lord's side, but her maternal side won over her evil one when she had to check if Harry was still alive in the Forest.
It was around this time that Riddle gave himself the alias 'Lord Voldemort', to spare himself of the reminder of his 'filthy Muggle father', who he later learned had abandoned his wife after learning she was a witch (although in reality he left her after she stopped controlling him with a love potion).
As Harry said, Dumbledore was the only person who could see right through Tom Riddle. He was already suspicious about what Tom did to the kids from orphanage and later to Myrtle and Hagrid. As one of his teachers, he knew Tom's tendencies toward Dark Arts. And yet he didn't do anything about them.
Why wasn't Hagrid sent to Azkaban? Because nothing was actually proved since Aragog escaped. And he did not murder himself neither did he order Aragog to do so, it was assumed that his pet might've attacked on its own.
