Zlorf Flannelfoot was a previous president of the Assassins' Guild. He is mentioned in The Colour of Magic, and described as having a face that is abysmally scarred, as if he's been in hundreds of fights. He also indulges in petty arguments and threats with the then-Head of the Thieves' Guild, Ymor, during a clash over a rich foreigner in the Broken Drum.
He was commissioned by the Patrician to have Twoflower quietly eliminated, based on a strong suggestion from the then-Grand Vizier of the Agatean Empire. Accompanied by several other Assassins, Zlorf went to the Drum only to find Twoflower in the custody of some Thieves who were planning to rob him. Seeking an alternative to a tavern brawl, Zlorf offered Ymor this deal: "I'll kill him, and then you can rob him." The conclusion of this deal was interrupted by the arrival of representatives from the Merchants' Guild, which was less than a day old. All three parties then began fighting over Twoflower. Presumably, he survived, but at least one of his Assassins did not. Although a city-wide destructive fire breaks out soon afterwards, from which others count themselves lucky to escape...
Prior to entering the Drum, he and his Assassins neutralized guards placed by Ymor outside the Drum, and some others in a doorway across the street... but forgot to remove two bowmen on the roof. A classic example of over-confidence.
This is an image of Assassins that TP has refined over the years, to the point where they are seen (at least by themselves) as refined gentlemen.
Conversely, we are also told that the Guild, in the noblesse oblige manner of the best public schools, offers a limited number of scholarships to poor and socially unconnected - but potentially very, very, talented - students. Witness Inigo Skimmer and "Professor" Cranberry. It is possible that Zlorf was such a scholarship pupil who advanced to the highest office?