Slipknot is a master in the use of ropes, including unbreakable ones, and a trained assassin. He created a chemical adhesive which he then applied to his ropes, making them nearly indestructible. Weiss uses the ropes to strangle, grapple, and hold down his opponents. In addition to his ropes, Slipknot is an expert assassin, able to kill swiftly and silently.
He is one of the early members of Suicide Squad in the comics and, at least within the comics, he is a formidable assassin (albeit, like most formidable assassins in comics, he very seldom succeeds). He is also mildly famous for being Captain Boomerang's testcase in "The Final Price" / Suicide Squad 9 (January, 1988) on whether the explosive leashes (then bracelets, not cranial bombs) actually worked.
Slipknot discusses the 'arm band bombs' with Captain Boomerang. These deterrent devices are placed on the less trustworthy members and are set to explode if the person goes too far out of range.
Captain Boomerang states that he believes the bombs are fake. This is enough for Slipknot. When he discovers his talent for strangling is useless against robots he makes a run for it into the swamp. The device begins beeping and flashing an alarm, but Slipknot ignores the warning and once he gets out of range, his right arm is blown off. Boomerang, who had not been sure of the bombs' existence himself, thinks 'better you than me'.
So, in short, it's a callback to continuity and a way to show that Waller is deadly earnest about keeping them in line. As a bonus, as a relatively obscure character, his death isn't likely to affect sales.