Daniel Leighton, also known as Cutthroat, is Diamondback's brother.
When his brother was killed by Brock Rumlow, Danny felt so embarrassed for not helping him that he left the USA. He wandered through Europe, got really good using blades, and became a mercenary under the name Cutthroat. He got a mission in America where he was almost captured by Nightcrawler and Spider-Man, so Cutthroat understood he was not ready for high-profile jobs. He returned to Europe until he found Red Skull needed a new lieutenant to replace Crossbones. Mother Night had Cutthroat get further training under Taskmaster, and after defeating other two candidated, Cutthroat got the job. Sick of the way Red Skull mistreated Mother Night, Cutthroat began a romantic relationship with Mother Night behind the Skull's back.
When Crossbones and Diamondback went to the Red Skull's mountain hideout in Colorado, they were "greeted" by Blackwing, Cutthroat and Jack O'Lantern. Crossbones threw a rope to Jack O'Lantern's ankle, and Diamondback jumped on his glider and threathened him with stabbing his heart with a poisonous diamond. This earned Crossbones and Diamondback an audience with the Skull. When Cutthroat heard Crossbones referring to Diamondback as "Rachel", he found out she was his sister.
The Red Skull allowed Crossbones back into the Skeleton Crew for a probationary period, and had Diamondback imprisoned. Cutthroat visited her in her cell and revealed his identity. He also told Rachel that he would kill Crossbones to ensure his place as the Red Skull's lieutenant.
That night, Cutthroat went to Crossbones' room to kill him, but Crossbones woke up. In the resulting fight, Cutthroat's sword fell and Crossbones picked it up, using it to cut Cutthroat's throat. Danny died not knowing the man who killed him was his own brother's murderer.