In Act 1, Scene 2 of The Tempest, both Prospero and Miranda berate Caliban for being completely evil by nature. When Caliban complains about being mistreated and confined to a desolate part of the island, Prospero replies:
Thou most lying slave,
Whom stripes may move, not kindness! I have used
Filth as thou art, with human care, and lodged thee
In mine own cell, till thou didst seek to violate
The honour of my child.
Caliban shows no shame or regret. He says:
O ho, O ho! Would't had been done!
Thou didst prevent me; I had peopled else
This isle with Calibans.
Miranda must have been quite young when this assault occurred. According to her father, she was three years old when they arrived on this island and they have been there for twelve years. So she is now only fifteen. Miranda remembers how Caliban tried to rape her and turns on him:
Which any print of goodness wilt not take,
Being capable of all ill!
She goes one to relate how she taught him to speak and taught him many other things before discovering that he was hopelessly evil.
But thy vile race,
Though thou didst learn, had that in't which good natures
Could not abide to be with
The best evidence that Caliban is a bad person is in the testimony of the only two people who know him, Prospero and Miranda. Throughout the play Miranda shows herself to be exceptionally gentle, kind and loving, so her testimony against Caliban is especially damning. Both Prospero and Miranda state that they tried to civilize him until they came to realize that it was impossible because of his evil nature. He was willing to learn but not to reform.
You taught me language, and my profit on't
Is I know how to curse.
Caliban is a bad person because he tried to get Trinculo and Stephano to kill Prospero.
Caliban is one of the inhabitants of the island, brought there when his mother Sycorax was pregnant. He has spent his life there and knows it well. If your job is to draw conclusions about Caliban, the conclusion that he is up to no good is a reasonable one to make.
If you are drawing the conclusion that Caliban is a bad person, and need evidence to support that point, there are several arguments you could use to support it. First of all, Caliban is never described in a very positive light. He is not treated well but he has done some bad things too. Prospero claims he tried to rape Miranda, and ever since then he has been treated like no better than a slave. For this reason, he takes advance of Stephano and Trinculo’s presence on the island to try to get out from under Prospero’s thumb.
Since they have liquor, Caliban is convinced that they are higher beings. Caliban swears an oath to Stephano, the butler, in exchange for the two of them killing Prospero. Caliban knows Prospero’s habits and powers well, and is quite specific.
Why, as I told thee, ’tis a custom with him,
I'th’ afternoon to sleep. There thou mayst brain him,
Having first seized his books, or with a log,
Batter his skull, or paunch him with a stake,(90)
Or cut his wezand with thy knife. (Act 3, Scene 2)
Unfortunately for Caliban, Stephano and Trinculo are hapless clowns and never get close. Prospero does find it in his heart to set Caliban free, despite the fact that Ariel has been watching everything Caliban has been doing and reporting it back. Propsero know Caliban’s plans, and Caliban never had a chance.
For all of his cursing and whining, most of the time he is pretty harmless. He can even be helpful, offering to show Prospero and Miranda how to survive on the island back in the days before the incident that got him banished to the role of slave. Nonetheless, Prospero and Miranda seem quite convinced of his ill intentions toward her, and he does offer her up to Stephano quite freely, telling him “she will become thy bed,” so there might be something to the allegations.