Saddam launched multiple gas attacks on the Kurds. He was trying to kill all the Kurds... Seems like genicide to me.
MS13 is definitely one of the deadliest. Watch the documentary on Netflix. Those are some crazy bad mofos. I shudder to think what would happen if they took over.
Yakuza wouldn't be considered vicious, their killing style is more efficient rather than 'spray your body on the wall and make a face lantern' type message. I'd say gangs from 3rd world countries are most vicious, they have to be.
The Jamaican shower posse was hardcore. I don't know if they are as strong as they used to be. The south American gangs are bad also.
Ms13 for street gangs. But out of the two groups Op mentioned, Los zetas for sure. Those guys are fucked up. "I'm to drunk, to taste this chicken" -Talladega nights