Is there any quote from Macbeth admitting that he wants to take Malcolm's place, or kill Duncan?
One of the odd things about the play is that Shakespeare sort of skirts the problem of Malcolm's succession after Duncan has been murdered by having both of Duncan's children escape to England and Ireland. Malcolm, in some way that Shakespeare doesn't explain, doesn't get elected to the throne because of this, and Macbeth is elected in his place.
Oddly, Macbeth never really says anything about taking Malcolm's place, specifically, though he does say things about taking the throne: here he is after the witches initial prophecies:
Glamis, and Thane of Cawdor!
The greatest is behind.
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme! ...This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth?
Macbeth believes the prophecies, knows that he wants to become king: he has "black and deep desires" to take the throne. And later, when he talks to Lady Macbeth, she says of Duncan that "he that's coming must be provided for". Macbeth knows exactly what she means.
It's nothing against Duncan, though, who Macbeth thinks has been a good king. Macbeth says nothing against him, or against Malcolm. He just wants the crown - for himself.