"you must act like THE MORE aggressive predator" is the use of "the more" correct in this sentence. thank you
Yes, that's correct.
Many adjectives in English can be modified by adding "-er" or "-est" to describe a comparison. For example, "bold" can be modified in this way: "You must act like the bolder predator." The following link describes some general rules for applying this modification:
HOWEVER, there are also many adjectives that cannot be modified in this way. I do not know of any rules that can help you know which is which -- some just can, and some just can't. "Aggressive" is one that can't, it is not correct to say "aggressiver" or "aggressivest."
For these adjectives, you use "more" and "most" to modify them, just as you do above. Your sentence correctly communicates that the subject must act like the predator who is more aggressive. (This of course assumes there are two predators involved. If there were three or more predators, then the subject must act like the predator who is most aggressive.)