Actually I think the first few episodes of The Untouchables had Nitti getting killed and other “happy endings” I guessed that the series was getting popular so they rewrote to include the gangsters on an ongoing basis
No, they killed him off in the 3rd season by having him run over by a subway train un NYC,
The REAL Nitti committed suicide rather than face prison for tax evasion in the late 30’s or early 40’s and was succeeded as boss of the Chicago “Outfit” by Charlie Fischetti.