Macbeth can’t be trusted because he pretends to do one thing while actually doing another. He also has a wife who is ambitious, and after he becomes king he has a tendency to become paranoid.

Macbeth pretends to be a loyal subject to King Duncan.  Lady Macbeth tells him to be sly and wily and no one will know the difference.

To beguile the time,

Look like the time; bear welcome in your eye,

Your hand, your tongue; look like the innocent flower,(70)

But be the serpent under't. (Act 1, Scene 5, p. 20)

Thus while Macbeth is pretending to be Duncan’s friend, and inviting him to his castle, he is really plotting to kill him. 

One of the reasons Macbeth cannot be trusted is that his wife is very ambitious, and really wants Macbeth to be king.  While he might have been harmless without her, she makes him dangerous.  When he falters, she pushes him on.

We fail?

But screw your courage to the sticking-place,

And we'll not fail. (Act 1, Scene 7, p. 24)

As long as Lady Macbeth is around, I would wonder about Macbeth.  She already got him to murder, what else could she get him to do?

Finally, after Macbeth kills Duncan he becomes even more dangerous.  He kills his friend Banquo, to prevent his sons from becoming kings and because he may know something or suspect Macbeth.  He also kills Macduff’s entire family.  He is definitely not to be trusted.