From The Collaborative International Dictionary of English v.0.48 :
Fault-finder \Fault"-find`er\, n.
One who makes a practice of discovering others' faults and
censuring them; a scold.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
faultfinder
n 1: someone who is critical of the motives of others [syn:
cynic, faultfinder]