She probably wanted to get Von Croy's attention. When she planned a trip for Egypt to get the amulet from Set's Tomb, she probably heard that Von Croy was in the area and was after the amulet as well. She probably remembered the first encounter being with Von Croy in Cambodia thus we have the first levels of TR4 and thought that stealing the Iris would be a good way of getting his attention and having him know that she was the one who did it.
Lara must have thought it to be a perfect opportunity to settle an old score with him.
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