Neon is the Colour of Vengeance by Elanor Miller

He loved. He lost. He killed.
The year is 2423, and the Jazz Age is in full swing—again.

Hugo Ford is the most dangerous man in Tenebrium City. A mafia hitman with a singular focus, born and raised to do one thing—and he does it very well. He completes every mission. He cashes every chit. He never misses and he never fails…that is, until he’s tasked with killing Evan Carter.
Inexplicably drawn to the man, Hugo finds himself unable to kill him. Never before in his life has he disobeyed one of his boss’s orders, but there is just something about Evan Carter. Forced to form an uneasy alliance, the pair strike a deal to save Evan’s life and cover up Hugo’s failings hoping to deliver them both from a grisly fate at the hands of Tenebrium’s mafia and Hugo’s own family.
Enemies at first, unsure of who to trust and where to turn, it isn’t long before they have to confront their feeling for one another. On the streets rival families war as the pair fall irrevocably, desperately, in love. But their secret won’t stay secret forever.