Recently in Bangladesh the opposing Tablighi factions were involved in murderous clashes in which a Muslim was killed and hundreds injured.

Answer

In the scenario explained by you, they did not become murtad. However, both groups are Jahannami. Rasulullah (Sallallahu alayhi wasallam) predicted this type of fitnah, and he said about the killer and the killed: "The qaatil (killer) and the maqtool (the murdered one), both of them are in the Fire."

Since both groups of the Tabligh Jamaat are on baatil, all of them are fit for the Fire - the killers and the killed. All of them have been trapped by shaitaan.