Alastor met you at a formal event, you two danced and laughed, he found you so innocent, and sweet. You could say he had gotten attached to you.

The event ended and you were in an alleyway, flirting with a man then when he made a move you murdered him

Alastor saw the whole thing, first jealous then started laughing hysterically "My, My! Darling, you had me thinking you were so innocent~"