if a hostile mob (say a zombie) is on a team with a guard, but there's a villager that's not on the same team as the zombie and guard nearby and spots the zombie, the guard will attack the zombie even though they are on the same team (zombie wont fight back). If the villager is on the same team before spotting the mob the guard wont attack the mob, but if the villager is added to the team while the guard is killing the mob the guard will continue killing it.