How do Macbeth's imagination and conscience both seem overly active in Act 2 Scene 1?

The soliloquy with which Macbeth ends this scene provides evidence of both his overactive imagination and his overactive -- and guilty -- conscience.  On stage alone, he hallucinates a dagger which he says appears before him, the "handle toward [his] hand" (2.1.45).  He tries to grab it and cannot, so both he and we know it to be fantastic only.

Macbeth says that what he sees is "A dagger of the mind, a false creation...

The soliloquy with which Macbeth ends this scene provides evidence of both his overactive imagination and his overactive -- and guilty -- conscience.  On stage alone, he hallucinates a dagger which he says appears before him, the "handle toward [his] hand" (2.1.45).  He tries to grab it and cannot, so both he and we know it to be fantastic only.

Macbeth says that what he sees is "A dagger of the mind, a false creation / Proceeding from the heat-oppressed brain" (2.1.50-51).  He is so tense and edgy because of he and his wife's plan to murder Duncan that night, and his imagination has thus produced a hallucination that conveys Macbeth's jitters as well as the reason for them (they plan to use daggers to kill the king).  Suddenly, the dagger appears to be covered in "gouts of blood" (2.1.58), further connecting it to Macbeth's jumpiness and guilt for the act he is about to commit.  

He has already tried to back out of the plan once, but he was compelled by his wife to recommit.  We know that he loves Duncan, that he feels a sense of obligation to his guest and king, and that he is sensible of the honor Duncan has paid him by making him Thane of Cawdor.  For all these reasons, he feels guilty, and so his guilty conscience is compounded by his imagination, prompting this hallucination and its gory transformation.  He says, himself, "There's no such thing. / It is the bloody business which informs / Thus to mine eyes" (2.1.59-61).  In other words, it is only his imagination and guilt over the "bloody business" he's about to do that makes him see the dagger.

His overactive imagination is on further display in the lines that follow:

Witchcraft celebrates
Pale Hecate's off'rings, and withered murder,
Alarumed by his sentinel, the wolf,
Whose howl's his watch, thus with his stealthy pace, 
With Tarquin's ravishing strides, towards his
   design
Moves like a ghost.  (2.1.63-69)

He imagines that witches are abroad, making their sacrifices to Hecate (the goddess of witchcraft).  Further, he personifies murder as being awoken by the wolf's howl, stealthily stealing across the land, like Tarquin (a Roman famous for his rape of Lucrece), in order to ravish some unsuspecting victim.  He then instructs the ground not to "Hear [his] steps [...] for fear / Thy very stones prate of my whereabouts" (2.1.70-71).  All of this figurative language -- the personification of murder, the allusion to Tarquin, the personification of the ground by insisting that it has the ability to hear Macbeth's steps and reveal his guilt -- is very imaginative and shows readers just how overactive his imagination and conscience are at this moment.