It is important to respect and be fair with your enemies. There are at least two dangers that arise from disparaging your enemies.
First, you will no longer evaluate them accurately, and this undermines your ability to strategize effectively. If you assume your enemy personifies every vice and negative quality (which is obviously irrational), you will fail to accurately anticipate their reactions and responses, you will not understand their motives and plans, you will be unable to confront or resist them appropriately.
Second, you may become satisfied with only disparaging them, instead of actually defeating them. The petty gratification you get from insulting them may appease your anger, and you will lapse from fighting them in a real way. This happens a lot, I’ve noticed.
It is not necessary to personally despise an enemy, it is only necessary to defeat him. Hate what has caused the enmity between you, hate their aggression, their injustice, their oppressiveness, but hate calmly and effectively.