The simple answer is that the king, Duncan, has decided to reward Macbeth's loyalty and courage on the battlefield by awarding him the title of one of the slain traitors--The Thane of Cawdor. Ross says:
The king hath happily received, Macbeth,
The news of thy success; and when he reads
Thy personal venture in the rebels' fight,
His wonders and his praises do contend
Which should be thine or his. . .
And, for an earnest of a greater honour,
He bade me, from him, call thee Thane of Cawdor. . .
This is a great honor, but it is more than that to Macbeth. It is this fulfillment of part of the prophecy just delivered to him (and Banquo) by the Three Witches. And it is this fulfillment of the first part of the prophecy that makes the idea of killing Duncan in order to fulfill the second part -- that Macbeth shall be king -- so possible in Macbeth's mind.
It is also ironic that Duncan bestows the title of Thane of Cawdor on Macbeth, since the slain Thane of Cawdor that Macbeth replaces was killed in rising up against his king, Duncan, in a failed attempt to gain Duncan's power for himself.