In this context from Mat 7, Jesus was referring to the wicked Jewish religious leaders of His day as bad trees. They were the ones bearing bad fruit. Prominent men in the Bible are often referred to as "trees" (Psa 52:8,, Isa 56:3) while the common man is called "grass." (2 Kin 19:26, Job 5:25, Psa 37:2). In Mat 3, John the Baptist tells these wicked scribes and Pharisees that the "ax is already laid to the(ir) root." Jesus repeats this theme in the Parable of the Fig Tree in Mat 21. He is foreshadowing what will happen to the wicked generation of those Jewish religious leaders who would soon put Him to death. In turn, they and their children, will perish in the War with Rome in 70 AD and indeed Titus had every last priest killed.