What were Macbeth's and Lady Macbeth's reactions to the murder just after it was committed?
In Act 2, sc. 2, immediately following Duncan's murder, the scene is somewhat chaotic because Macbeth and Lady Macbeth are both nervous and in a heightened state of awareness. Macbeth says he heard many sounds, mostly voices. He heard someone in his sleep cry out, "God bless us!" and another cry out, "Amen!". When Macbeth himself wanted to say, "Amen" in return, the words stuck in his throat. The significance is that he has just killed a king and he has broken his connection to God and so cannot say the word. Macbeth also says he thought he heard someone say, "Sleep no more! Macbeth does murder sleep". The significance of this is that Macbeth suffers from insomnia ever after this. Macbeth also heard, "Glamis hath murdered sleep, and therefore Cawdor / Shall sleep no more! Macbeth shall sleep no more!" Macbeth has come into Lady Macbeth's chamber holding the bloody daggers and with blood on his hands. He is in a state of shock and seems remorseful. Lady Macbeth, who appears much calmer and cooler, chides him for forgetting to leave the bloody daggers on the guards and takes the daggers to place them on the guards in order to implicate them in the murder of Duncan. She tells him she'd be ashamed to be as remorseful as he is and that a little water cleanses them both literally (as it washes off the blood) and figuratively. In Act 5, she will try to wash blood from her hands.
Right after the murder, Macbeth is more unstable that Lady Macbeth. In fact, Lady Macbeth reassures her husband that a little water is all they need to cleanse themselves of the deed. However, Macbeth does not agree, he says: "Will all great Neptune's ocean wash this blood clean from my hand? No, this my hand will rather the multitudinous seas incarnadine, making the green one red" - (Act II, Scene II).
Macbeth is totally unhinged after the murder, he is descending into paranoia. Lady Macbeth remains more stable initially.