Annexation brings us closer to manifest destiny. The United States is one step closer to having California as apart of their states.
War with Mexico
Mexico does not recognize Texas as apart of the United States. Which will lead to war with Mexico
From a free Black Man's point of view, "When Texas was a Republic I could have slaves but when Texas got annexed by the United States I will lose all my individual rights." Slavery is a good thing for 2 reasons because 1 slaves can work on farms and 2 you could send them to war.