Easy. First, controversial positions on certain issues are not necessarily associated with antipathy towards the opponent. It is necessary to distinguish aggression towards opinion from aggression towards the individual. It is bad form to get personal in a dispute about opinions.
Secondly, in some cases, an aggressively ironic manner of communication is adopted between friends (this is often the case with boys - for them it still has the character of such a playful pissing) .
Thirdly, some people have peculiar ideas about intimacy. For them, the closer a person is, the more harsh jokes can be made in his address and allowed in his address. At the same time, mutual aggression in an argument carries a connotation of sympathy. Something like "you are my friend, so I can tell you that you are an asshole." =)
They can, especially when it comes to people who are in frequent contact in real life. In this case, aggressive communication can be part of the style, line of behavior, joke in the end.
Best friends, and large friendly companies often joke and ridicule each other (among the younger, they are often simply groundlessly insulting), not investing in this no real anger, and realizing that they are all the same friends.
From a more mature example - the State Duma, where "irreconcilable" opponents, after turning off the cameras, become bosom comrades, drinking and building business together ... condemning each other also via the Internet - this does not prevent them.