Another theme could be the power of temptation. The 'darkness' represents something that is so alluring to the character that they are willing to give up love for it. It might be a symbol of a forbidden pleasure or a dangerous path that seems more exciting than a stable relationship.
A third theme might be regret. Even though the character has chosen 'darkness', the fact that they start with 'I love you' implies that there will be some sense of loss or longing for what could have been. They may later come to realize the value of the love they sacrificed.
One possible theme is the conflict between love and self - interest. The character's choice of 'darkness' over love might suggest that they prioritize their own needs or desires above the relationship.