MGSV has a lot of symbolism relating to butterflies, specifically the Morpho butterfly. Ignoring Quiet, butterflies (iirc) only show up in two places in the game: After learning the truth about Paz and in the famous hallway scene. In the Paz truth scene, phantom Paz leaves behind a single Morpho butterfly as the only proof of their existence (Or, non-existence.). Blue butterflies have two major meanings; The first is happiness. If you see a blue butterfly, it's common superstition to make a wish on it; The other... is the passage of time. The name Morpho means "change".
Venom crushes this butterfly.
I love this moment due to the sheer amount of symbolism going on in just a couple seconds, all done without dialogue. Characters crushing flowers/killing small parts of nature without a second thought symbolizes the loss of innocence. Him specifically crushing a butterfly that symbolizes hope and good fortune helps play into his transformation into a demon. There's no room for angels in his heaven, after all.
But the second meaning, the passage of time, tells a different story. He doesn't want things to change.
And in the context of this image, Kaz is looking back on the past. Back when things were pure.
Back when he still had the luxury to believe in things like good fortune.
