Couple of things that people seem to be missing:
The officer says that they only saw a metal object, not a pen, and that being said you can still do a lot of damage with a pen. Getting cornered by a guy in a wheelchair isnt as stupid as it sounds depending on room layouts, other people in the room, household objects, etc.
Im not saying fatal force was correct, but it is easy to sit in a room over a cup of tea/coffee & say what you should do, and discuss it, but when you are there, and the amputee is screaming at you and potentially armed with an unknown object - rash decisions can be made.
Sometimes it is the wrong decision, and those are the ones we hear about in the news.
When its the right one you hear nothing in the news.