Does Bad Things Guy was a character in the SML series who was the fill-in for Brooklyn T. Guy from 2016-2017, as well as his and Doctor Me’s youngest triplet.

He was also the main antagonist of his final appearance, "The Secret Door!" where he was revealed to be a true villain all along. Having murdered countless people (including Bowser Junior in "Pinch! Pinch! Pinch!", although this death is non-canon), he almost succeeded in murdering Mario, Rosalina and Jeffy, before being shot in the head by his brother.

More facts here: https://sml.fandom.com/wiki/Does_Bad_Things_Guy