How unfortunate. Of course we do not know the complete details, standard procedure, and protocols of the marshalls so we can only speculate.
That being said, I see no wrong on the marshalls part. Extreme, yes, and maybe he could of used an escalating amount of force. Some may say since the man was off the plane, there was no life threat.
To say it is extreme, well good for the marhsalls. This is a prime example, and reinforces the message 'Don't f--- around at the airport.' It proves that they are there to protect, and capable of doing so if needed. If a bomb was found, this marshall would be a hero.
To argue that the man was no threat after he exited the plane is a moot point. A plane can be blown up from both the inside and outside. He was still a threat in that regard.
I have to wonder what a mentally ill man is doing on a plane without taking his medication. I am quite stable (I hope), have jumped out of perfectly good airplanes, and I still experience anxiety when I fly. This man and his wife were aware of his condition and should have taken precautions.
Maybe the marshall was trigger happy and really screwed the pooch on this one. Maybe he was just doing his job to the best of his ability.
I do not support or cheer for the death of an innocent man. I just feel the need to side by the marshall, a man like myself who is sworn to protect. It will be easy for some to bash him, but unless you have been in a situation that requires split second life or death decisions, think twice before blaming the marshall.