Doomsday by Warwick Deeping

But avoidance was not part of the play. She was like a spoilt child in a menagerie coquetting with a lion that was safely caged. All her animals were caged, and you could throw buns to them, and say--"Pretty, pretty," and remark to your nurse that the lion looked cross. It did not occur to her that she might be an offence and provocation to some wild beast behind the bars, and that man may be a very dangerous wild beast, more especially so when he has been teased and thwarted by fate, and has in him the makings of a fanatic. And man is a reasoning animal, a creature with ideals that may have stern eyes, and a fantastic frown on its forehead.

They took the Melhurst road together under the frayed gold of the afterglow. He was rather silent, not looking at her, but listening to all she said as though he were weighing every word of it and estimating its sincerity. He moved with long strides, shoulders slightly bowed, while she went boyishly with her shingled head, and her slim legs swinging under her short black skirt.