In Macbeth, is Macbeth a victim or aggressor?

Both.  It does seem as though, in the beginning, the Weird Sisters plan to manipulate Macbeth.  When they meet in Act 1, scene 1, they chant, "Fair is foul and foul is fair / Hover through the fog and filthy air" (1.1.12-13).  They appear to imply that they are going to make good things seem bad and bad things seem good, and this is designed to trick Macbeth into believing that something is good when it will really lead to his ruin.  In this sense, then, he is a victim.
On the other hand, once Macbeth's terrible ambition is sparked by the Weird Sisters' "prophecies," he becomes an aggressor.  He not only murders the king, Duncan, but he also plans the murder of his former best friend, Banquo, as well as Banquo's young son, and Macduff's innocent wife, children, and servants.  Once Macbeth starts on his path of destruction, he continues to become worse and worse, changing from a somewhat sympathetic figure to an awful and ruthless tyrant.