Well, that's complicated, but on the law enforcement side...
a) Death Row for violent felonies involving the use of deadly force, whether or not anyone is actually killed. By pointing a gun or knife at someone and demanding something you are in effect implying you're willing to kill them, and that's enough for a called strike three IMO. In other words, if you use deadly force in commission of a violent felony you die.
Most homicides are impulsive, committed under the influence of drugs/alcohol, in the heat of passion, or by individuals with mental illness, groups that do not engage in rational cost-benefit analysis. Even premeditated murders are usually committed by people who believe they won’t get caught (the clearance rate for murder is only ~60% in the U.S. A clearance rate is the percentage of specific crimes solved in a given year meaning, the odds are pretty good you'll get away with murder).
The vast majority of studies on the effectiveness of capitol punishment as a deterrent to crime show that it has virtually zero effect.
Capitol punishment for threats and attempts would violate the 8th amendment prohibition against cruel and unusual punishment.
b) Street gangs should be labeled domestic terrorist organizations along with the cartels, and then sic the feds on them rather than law abiding hard working DACA kids and other law abiding immigrants (currently a better use of resources along with securing the borders, and providing timely immigration services).
...though the economic side is another issue entirely.
The only criminal gangs I can think of that somewhat fit the definition of "Domestic Terrorists" would be: The Base, Atomwaffen Division, Proud Boys, certain white-supremacist (Aryan Brotherhood, Ku Klux Klan, etc.) and eco-terrorist groups
Here is the federal government's definition of Domestic Terrorism:
Primary motive: Ideological, political, religious, social, or racial goal intended to influence government policy or intimidate/coerce a civilian population
Intended audience/effect: Designed to influence or coerce the government or a broader civilian population through fear.
Statutory definitions: 18 U.S.C. § 2331(5) and FBI/DHS definitions require acts dangerous to human life that appear intended to:
(i) intimidate or coerce a civilian population;
(ii) influence government policy by intimidation or coercion; or
(iii) affect government conduct by mass destruction, assassination, or kidnapping
Lastly, criminal gangs are primarily investigated and prosecuted at the federal level already. Gang criminal activity is covered under RICO 18 U.S.C. § 1961–1968, VICAR 18 U.S.C. § 1959, Drug Conspiracy 21 U.S.C. § 841, 846, Hobbs Act Robbery / Extortion 18 U.S.C. § 1951, Continuing Criminal Enterprise (CCE) – “Kingpin” statute (21 U.S.C. § 848), Firearms offenses (18 U.S.C. § 924(c)), Transnational Organized Crime designations.
Federal law enforcement agencies involved are: FBI, DEA, HSI, ATF, Treasury, DOJ + U.S. Attorneys.