The poem starts out lightly. As a child, the speaker kept inventing careers and asking their mother if they could do them, and she always answered, "You can do anything." It sounds like a warm, familiar memory of a parent's encouragement.
Then the tone shifts. Speaking directly to the mother, the speaker admits that wanting to be a detective had nothing to do with solving crimes. The reason is far more disturbing: when "he" hits them and they taste metal in their mouth, they realize that blood excites them, and so do the dead. The poem doesn't say who "he" is, so the reader is left to imagine.
The mood is quiet, unsettling, and sad. The short, plain lines make the confession feel flat and numb, as if the speaker is stating a hard fact. The mother's reassurance now feels painfully ironic, since her words of hope have met a very different reality. The poem suggests that violence has reshaped the speaker's desires, and it leaves us with that uncomfortable thought.