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He and one of Hugo's exes (the b*tch Falcon) conspired to drug Lucia and destroy her reputation during some celebrations in the imperial castle. Sir Crotin found something suspicious about all the situation and to protect the duchess simply killed everyone involved - Falcon, Ramis, all the knights and maids who where witnesses that Lucia even was there (as he said "it was safer to first kill and then take care of the consequences).
Well i skipped the large part of the novel, where Vivian was in the capital - i believe somewhere from coronation and till this Mad Dog's shining moment. Somehow it was too boring to read, I just skimmed through. But later it began to feel interesting again, there will be
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academy life of Damian, Vivian's pregnancy, Hugo's past, the history behind that necklace that showed Lucia's future, the erased future after she became commoner, their life when a cute daughter is born and so on. It deserves to be read till the end (=・ω・=)
This is why he was killed lol jk I forgot why he was killed but I remembered he never got over his arrogance and was killed by the duke and it somehow involved Vivian.