What would you say about a mentally unstable metahuman named Hector (this pun is so good it's completely retarded, yes), whose power is to immediately notice and gradually amplify one of three biggest flaws in any sort of system he concentrates on?

For example, the character notices a minute, tiny crack or chink in the load-bearing wall or a piece of armour, concentrates on it and the crack begins to grow and widen really quickly until the wall collamses, while armour begins to rust over and enlarge it's vulnerable spot until it reveals all of the enemy's back.
 
Or, for example, this power may allow the character to notice that while a martial artist's style is all-around strong and capable, footwork receives the least amount of attention, and he worsens that flaw until the aforementioned artist literally can't stand properly, tripping over his own feet - or if in the mental composition of this man, fear or anxiousness are strongest negative emotions, they will worsen and gradually grow stronger and stronger until he drops everything and runs away crying in terror or just stands in place completely unsure of what to do and how to act even if his life depends on quick thinking.

The first weakness i have in mind is that as he worsens the flaws of others, his own negative traits amplify accordingly, until he suffers a major mental breakdown or temporarily goes completely nuts in a different way. Maybe he can also be defeated by ceasing to rely on one's flawed aspects or parts of personality - i.e, in case of the aforementioned martial artist, completely disregard the footwork and fight using some boxing techniques.