Julius Caesar did pave the way for Rome to almost completely abandon the whole point of the Republic when he was named dictator perpetuo, and later when Octavian became emperor. They're both responsible for doing some rotten things but you may think that many of the emperors that followed were much worse.
Tiberius
This was Octavian's choice for successor, however it wasn't his first. Tiberius, in fact, didn't even want to be emperor. He was forced to divorce his wife and marry Octavian's relative. His mental health deteriorated later on and he became so paranoid he held trials on senators where they were executed, claiming he had secret files on everyone. Eventually he moved to his villa on the island of Capri where he engaged in horrifying activities. There he had a bath that he stocked with young children for his sexual pleasure. When he tired of them, he threw them off the cliff. He was absolutely crazy and debauched and for that the Romans hated him, but they couldn't get rid of him. So much they despised him that once the people learned he was dead, they collectively rejoiced at the news.
Caligula
Chosen by Tiberius as successor, he was raised at his villa where he was forced to engage in these acts. It is famously claimed that Caligula smothered Tiberius to death, but there is not much evidence to prove this. Partly due to Tiberius' poor reputation, he was actually chosen and confirmed by the Senate as the next emperor. Another reason for this decision is because Caligula appeared before the Senate stating his intention to destroy the secret files. For about a year he seemed quite rational, until he fell ill with a terrible fever. Many accounts state that after he recovered he was never the same again.
For instance, he really loved one of his sisters, and once she was pregnant he ripped the fetus out claiming he couldn't wait to see the baby, killing them both. He did indeed name his horse Consul but this was more likely a display of power and that he could do anything he pleased. He even built pleasure barges and drained the treasury on liquor and women. Ultimately, people felt they had to get rid of him. After leaving an event through hidden corridors, his own body guard killed him and his wife and child; ensuring their were no direct heirs. Once this occurred, it was difficult to find another emperor since most had already killed each other off.
Claudius
Although he doesn't quite compare to the rest, I thought I'd include him just because his story is tragic. Claudius was the only person left because people thought he was mentally challenged. He had some sort of disorder where he drooled on himself and so he was often thought of as a fool. However he was actually well-educated. Once he heard that Caligula was dead, he hid away fearing that he would be next. When the Roman soldiers discovered him, he begged to not be killed, but they were there to announce his title. One achievement we can attribute to him is his conquering of England, although sadly he was poisoned by his wife because she wanted her son, Nero, to rule.
Nero
A megalomaniac, he thought quite a lot about himself and that he could do just about anything. He owned many trophies for events he didn't even compete in. For instance, he had all the trophies for every Olympic event. He didn't rule very well, he even went so far as taking the treasury just to build himself a large home. His behavior was very odd and it consisted of him doing things such as sneaking out at night pretending to be poor and robbing others. At the time, people even thought he caught Rome on fire. Also, he did indeed burn Christians alive. The Senate finally decided it was time to get rid of him as well so they rose an army to fight him. Nero initially tried to flee to the east but they were able to catch up with him. Before he could die by Roman hands, he chose to commit suicide by servant, where he was stabbed in the throat.
