Shakespeare's Sonnet 149 is seems to be directed towards the same woman that is addressed in sonnets 127-153, and to get the entire story from this one sonnet is difficult without knowing somewhat of the setting of the other sonnets (both before and after 149).
For centuries, and still today, the theme of Shakespeare's sonnets was presented as a complex set of stories concerning Shakespeare's own relationship(s) with that rival poet, his fair-haired friend, and a dark women, whom he either served, was in love with, or was angry with because she was pursuing his friend.
Sonnet 149 continues the theme of the other sonnets by speaking to the dark lady. The sonnet seems to be both apologizing for the speaker's actions, as well as explaining those very same actions. The tone of the poem is apologetic as well as abject. It's a confusion shared by the speaker as well as the reader of the sonnet
The tone is set by the first couple of lines of the sonnet by showing how subservient to his mistress the speaker has become, even to the point where he is going against himself or his own thoughts, by loving her "for her sake". The speaker continues on by saying that he will hate all those whom she hates, and that (though he considers her a tyrant) will punish himself if he does wrong by her.
The speaker seems to have sunk so low that he does not even know, or understand, his own subservience (thy service to despise). In the very next line of the sonnet, he says the best of him worships the worst of her. ...