Though the desire to eliminate a witness is possible, I get the impression that this crime was committed due to a “perfect storm” based on mob escalating violence and mob mentality. The boys, either operating in that specific group, or in loose coordination with others are suspected in several mugging in the area. None of these victims were killed.
When confronted by the boys, Tess evidently refused to hand over her possessions. The probably agitated attackers then resorted to a choke hold (evidently a common NYC mugging tactic).
Tessa, perhaps fearing a sexual assault, then resisted further and bit the finger of one of them. Now the attackers were enraged. They could break off the mugging and risk humiliation (girl fought them off). Or, escalate the violence and “win” the encounter.
Apparently two of them escalated violence and stabbed the victim to death. Perhaps their dual presence “egged each other on”. The boy in custody is said to have been ten feet away when the stabbing occurred. He may have been trying to distance himself when the mugging did not initially go as planned.
In short, I think the murder was not planned as a way to prevent the witness from testifying. Rather, it was impulsive based on rage that the victim was continuing to resist and they were at risk of “losing face” if they "lost" the encounter.