Monster T was a crime boss in Gotham City who frequented the lavish strip club of the Joker. However, after lusting for the remarkably beautiful Harley Quinn, Joker, being a jealous boyfriend, promptly kills him.
Meeting with the Joker
At some point shortly after getting sprung free from Arkham Asylum, the Joker, a fellow crime boss ally of Monster T, obtains a lavish strip club. Monster T visits it one evening, and is greeted by the Joker, and though the latter refuses to shake his hand (as per usual, as Joker's right-hand man Jonny Frost reveals), Monster T welcomes him back, congratulating Joker on his escape.
However, Monster T then notices Joker's beautiful pole-dancing girlfriend Harley Quinn, and he promptly makes remarks about Harley (calling her a "bad bitch") and stares at her with lustful desire, greatly displeasing the Joker. But Joker initially hides it and proceeds to bring Harley over, intimidating Monster T, and making him somewhat uncomfortable by suggesting he sleep with Harley. Monster T, having picked up on Joker's angry sarcasm, attempts to assure the Joker that Harley is the latter's girl, and that he wants "no beef". The Joker, however, sees this as the mere pathetic pleading of a luster, and promptly shoots Monster T in the head.
Monster T is somewhat of a gullible simpleton, given how he openly lusted after Harley Quinn despite Joker's reputation as a very jealous boyfriend, as well as the ease with which Joker manipulates Monster T into talking himself into a trap, with discreet angry sarcasm.