Okay, this is pretty gruesome, but hey, you're writing a mystery! There's been a murder. Do these hands belong to the murderer or someone else?
Remember, this is a flashback. How old and where is the character when he or she has the flashback? What evokes the memory? How does he or she feel about these hands and all the blood? What does the character feel when he or she comes out of the flashback? Has her perspective changed? Is he in a different emotional place when the flashback is over? In this way, the flashback affects the plot.
If this image feels too gruesome, go for it anyway. Especially if you are writing or want to write a mystery. As writers, we have to be open to all areas of the human condition.
As Faulkner said, "The problems of the human heart in conflict with itself... alone can make good writing because only that is worth writing about, worth the sweat and the agony."
A final point to remember when thinking about flashback as an Ingredient in your mystery scene: In terms of technique, once you're in a flashback, the other four ingredients of the scene all come into play.