在主句为一般过去时的句子中 as though 引导的状语从句如果用虚拟语气谓语可以不是 have done 而是 did 吗?
But what else could they do on that dark winter's day when the last of their store of grain was gone, when the trees were cut and sold, when he had sold everything, even the little they had saved from the flood, and there was nothing left except the rafters of the house they had and the garments they wore? Was there sense in stripping the coat off one's back to feed one's belly? Besides, the beast was starving also, since the water had covered even the grass lands, and they had had to go far afield to gather even enough to cook its bones and flesh. On that day when he had seen the faces of his old parents set as though dead, on that day when he had heard the crying of his children and seen his little daughter dying, such a despair had seized him as made him like a man without his reason, so that he had gathered together his feeble strength and he had done what he said he never would; he had taken the kitchen knife and gone out and killed his own beast. When he did it, even in his despair, he groaned, for it was as though he killed his own brother. To him it was the last sacrifice.
(出自赛珍珠《贫瘠的春天》)
请问老师这一段落中的 killed 为什么不是 have killed 呢?回忆所学内容,主句为一般过去时的时候,as though 引导的状语从句假如不符合事实,后面的谓语动词应该为 have killed ,但是文段里面不是。这是为什么呢?这样的句子符合语法吗,还是某种特殊用法?请老师指教。