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Corax, the Raven Guard Primarch, went through a catastrophic moment like this. After the Isstvan drop site massacre, most of his legion was decimated, so he ordered some questionable methods to rebuild the legion. His orders left a lot of the recruits as mutated monstrosities. He personally Mercy Killed them, locked himself away for an entire year praying for their souls and his, and in the end, he took his ship and flew it straight into the Eye of Terror, his last words being simply "never more".
Part of the meltdown of Acro in Phoenix Wright: Ace Attorney. You don't actually see the moment onscreen but the aftereffects are obvious during his breakdown. In attempting to murder Regina he accidentally killed her father, to whom Acro and his brother owed everything. As Sympathetic Murderers go he's high on the list.
In Phoenix Wright: Ace Attorney - Dual Destinies , in the DLC case, Marlon Rimes is quite similar to Acro. His girlfriend died during a performance a year ago, and he believed Orla the orca whale was responsible, not knowing that 1) His girlfriend had a heart condition and 2) it was an entirely different orca. He tried to kill Orla by draining the water, but Jack Shipley, the owner of the aquarium and his boss, rushed out to save her, slipping and falling in the empty pool in the process despite Marlon's attempt to save him. Marlon tried to frame Orla for Jack's murder, but ultimately realized the truth of things and had this reaction. Thankfully, after it's pointed out that he tried to save her, he decides to make amends, however long it takes.
Archer from Fate/stay night as revealed in UBW route. He is trying to kill Shirou, himself from past, because he wants to prevent him making a pact with world to protect people, because it'll only make him one who kills some to save many.
One of the extended bad endings in the PS2 version of School Days has Kotonoha murder Sekai in cold blood and then offer herself to Makoto, who was watching. Makoto's horrified reaction causes Kotonoha to realize what she has done, and she breaks down in tears.