In Shakespeare's play Julius Caesar, it is indicated that Julius Caesar might become a dangerous ruler if allowed to live. There is no doubt that Caesar is an ambitious individual. He is currently a powerful ruler and likely wants more power. I mean he even compares himself to the North Star for crying out loud. He also must be quite charismatic for the common people to love him so much. Early in the play he parades down the streets, pretends to faint, all while being cheered and whistled at by the crowd.
None of that makes him immediately dangerous to Rome though. He's a popular politician. The conspirators fear what he may become. He may become king, which is a problem for a democratic republic like Rome. It would turn it into a monarchy and Brutus and his fellow conspirators are not happy about that.
He would be crown'd:
How that might change his nature, there's the question.
It is the bright day that brings forth the adder;
And therefore think him as a serpent's egg
Which, hatch'd, would, as his kind, grow mischievous,
And kill him in the shell.
They see Caesar as a dangerous unborn snake that is not dangerous now, but definitely capable of causing great harm after its birth when it reaches maturity.