Good day Grace777,
While I do agree that God does not kill believers because of their sins, I do believe that God killed these two. And the reason I say this is that, how could Peter know that they were both going to die? The reason would have been because the Holy Spirit told Peter, which would infer that it was God who killed them. The other reason and main reason I believe is because of the following in red:
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But Peter said, “Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back some of the price of the land? “While it remained unsold, did it not remain your own? And after it was sold, was it not under your control? Why is it that you have conceived this deed in your heart? You have not lied to men but to God.” And as he heard these words, Ananias fell down and breathed his last; and great fear came over all who heard of it. The young men got up and covered him up, and after carrying him out, they buried him.
Now there elapsed an interval of about three hours, and his wife came in, not knowing what had happened. And Peter responded to her, “Tell me whether you sold the land for such and such a price?” And she said, “Yes, that was the price.” Then Peter said to her, “Why is it that you have agreed together to put the Spirit of the Lord to the test? Behold, the feet of those who have buried your husband are at the door, and they will carry you out as well.” And immediately she fell at his feet and breathed her last, and the young men came in and found her dead, and they carried her out and buried her beside her husband.
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It was because they both lied to the Holy Spirit. I would have to say that the odds of both of them falling dead one after another and Peter knowing about it ahead of time would favor the Holy Spirit as the one taking their lives, otherwise, the odds of Peter knowing that was going to happen would be greater than hitting any lottery. But again, I believe that it was because of their lying to the Spirit.
Just my two cents worth ....