"Silver scale" seems to be a reference to a kind of shiny (silvery) body armor. "I wasn't a walking death trap with the mental agility of a beer keg." A "walking death trap" is a person who is rather "stupid". He is not a clear thinking person, and because of that, he is destined to die. "Mental agility of a beer keg". This is a week comparison to begin with. Beer Kegs are not objects which function with intelligence to begin with. Moreover, if one is drinking beer, the alcohol is going to make one function with less mental agility. The sentence is an example of poor comparison and a confused use of figurative language. It is not surprising you needed to have the sentence explained. If I found such a sentence in a book I was reading, I would stop reading the book as something written by an amateurish writer.

Breaking down "walking death trap": A trap is used to catch something, like mice or enemies. But this trap will only catch death, it will cause the death of the user. Worse, the trap is not in one location, it is walking around, so anywhere it goes, people will die. So you want to avoid someone who is like a "walking death trap" because anyone with them will die!