No one has downplayed it. It's been stated many times that they were in some kind of fight with other people in the airport and then involved somehow in the altercation with the police. Not fully sure about the full extent as details were scarce , but no one thought they were innocent.
The issue is that what happened before is irrelevant. The head kicking and stamping did NOT happen during the assault on the police . It happened after, once the man was tasered and lying flat on the floor. It wasn't an act of policing or retraining or arresting. It was violence and revenge and that is not acceptable.
The police are not judge and jury. They don't get to decide what criminals deserve based on their feelings on the day. That's not how this works.
Once you're ok with this for criminals, then you're ok with it for innocent people. Mistakes happen, mistaken identities, certain conditions can easily be mistaken for drunk/belligerent/threatening/aggressive. Crimes and what constitutes a crime differ, like putting stickers in a public place, making a twitter comment or calling a police officer a lesbian. People have been arrested, manhandled,taken to a jail cell for that.