Moral Event Horizon: Dr. Yung orders Mewtwo to destroy Mew. What makes this event notable is that Mew was the only Mirage Pokémon capable of sentience and compassion. In many ways, it was the perfect Mirage Pokémon, but Yung was incapable of seeing that. He only saw a failure because it wasn't perfect in his mind, a mindless weapon with absolute loyalty.
After creating Mirage Mewtwo, Mirage Master orders it to attack the other Mirage Pokémon. It does so and they are all wiped out. What makes this sad isn't so much that those Mirage Pokémon where destroyed (they really had no will or identities of their own). What makes it sad is Mew's reaction, it floats over to where they were all standing a moment ago and looks around with a mixture of confusion and sadness. Like it actually cared for its holographic brethren, and like a child can't fully comprehend what's just happened to them.
Mirage Mew's first apparent death, and its real demise later.