Why did Faber kill Mrs. Garden?

Faber killed Mrs. Garden because he feared that she would expose his spy activities.
In the book, Faber is a German spy working on behalf of the Axis forces. He disguises his spy activities by working as a railway clerk by day. At night, he retires quietly to his rented room in Mrs. Garden's home. For her part, Mrs. Garden is a lonely widow who has her eyes on Faber as a love interest.
One night, she decides to visit Faber in his room in order to seduce him. Deep at work transmitting information to German authorities, Faber is unprepared for his landlady's entrance into his room. Since his radio transmitter and code books are out in plain sight, Faber decides to kill Mrs. Garden.
When she swoops in for a kiss, Faber stabs her in the back with his stiletto knife. Eventually, he uses the stiletto to slash her throat. After killing Mrs. Garden, Faber leaves the house for a new location.