Urban street homicide varies sharply across regions, yet its explanation cannot rest on national homicide rates alone. The study compares Latin America and East and Southeast Asia to explain why lethal violence in public and semi-public urban spaces becomes highly concentrated in some cities, while similar pressures in others do not produce comparable levels of street homicide. Using a comparative qualitative criminology approach, the analysis focuses on the relationship between spatial concentration, urban marginality, and security governance. The argument developed is that street homicide becomes more likely when marginal urban spaces are linked to armed illegal markets and weak legal protection. In many Latin American cities, homicide is concentrated in specific blocks and streets where criminal governance, firearms, and impunity turn ordinary disputes into visible and repeated lethal violence. In East Asia, and in parts of Southeast Asia, similar urban pressures are often constrained by stricter firearm control, stronger offender traceability, and more predictable criminal justice responses. Southeast Asia remains more fragmented, as violence may shift toward borders, ports, security operations, or political conflict rather than appearing as conventional urban street homicide. The study contributes to comparative homicide studies by moving the analysis from regional averages to micro-spatial mechanisms and the capacity of law to interrupt retaliation before death occurs.