They were wise to fear. The story of Ananias and Sapphira illustrates a recurring theme in Luke's writings, in his gospel as well as the Acts of the Apostles. The recurring theme is hypocrisy and God's outrage in the face of it. When our Lord came upon the calculated deceptions of religious phoneys he denounced them on the spot. Few things provoked his rage like the calculated connivings of the cutesies.
Any sincere person falls short.
And for all sincere people who fall short our Lord has the tenderest word of mercy. But falling short of godly aspiration is as far from calculated duplicity as the east is from the west. Our Lord leaves no doubt of this at all. When Jesus was tempted or tested in the wilderness he refused to throw himself off the highest point of the temple and see if he would land on the ground intact. Christians love God.