Captain Beatty can be considered an antagonist and, if you want to get fancy, an apologist for the system. The “system” is a society that insists upon conformity from all its citizens and Captain Beatty aligns himself with this system.
The actions and attitudes that show Beatty as an antagonist are:
- Beatty’s threats (to Montag)
- Beatty's defense of conformity.
- Beatty’s affiliation with the system which the novel presents as evil.
To generalize Captain Beatty’s character, we can say that he stands as a representative for the social system which insists upon “equality” and which shuns those who choose to read or think for themselves. When Montag kills Beatty, he is completing his rebellion and making a final choice to break from this social system.