I am hoping that you are referring to D.H. Lawrence's poem, "Piano". If not, please disregard this answer! First, the speaker is feeling various emotions as the poem progresses. He is nostalgic for the past; the music from the piano transports him back to his childhood, where he is once again a child sitting underneath the piano in his home. He experiences longing for his childhood, and is sad that it is over. He also seems to grow angry and betrayed at the pianist because she is responsible for taking him back into his childhood and for causing his present strife.
This brings me to the second half of your question, about the speaker's use of the words "insidious" and "betray". I believe the word "insidious" is used to show his anger that he is being brought back in time. "Betray" is used here because he is fighting to stay in the present, but the music "betrays" him back into his past, or perhaps he has betrayed himself. I hope this helped you to better understand the poem!