"The worst student" is the student who is bad at things. In this case, "worst" simply describes the noun.
Following this logic, your "worst enemy" would be the person who is very bad at being your enemy, thus is actually your best friend. Your "worst enemy" is actually the enemy who is very good at being your enemy. Thus, in this context, "worst" acts to amplify the inherent negative intent of the noun. "Worst disaster" would be another example of this.
Is this just a matter of context or there some science behind the interprettation of a negative adjective applied to a negative noun?