If you are referring to "So is he mine . . . For sundry weighty reasons. (III:i)", I believe that Banquo's hopes and ambitions as expressed in act 4 make him a less sympathetic character. Be aware that in Macbeth's aside in lines 48-53, he admits that he hopes the king will be murdered. His ambition, it is represented here when Macbeth is talking to himself about his plans to murder Duncan, so that he can claim his title... therefore, i believe what i have stated above. I hope this helps! <3