In Dyalog APL, which isn't K:

          A←⊃⎕nget 'C:\test\a.txt' 1
          B←⊃⎕nget 'C:\test\b.txt' 1
          A~B
    ┌─────────────────────┬──────────────────┬────────────┐
    │AAAsofxxxLqKbYOAwLMHG│AAAdjLalKFapoYAcGE│AAAthGazmnfK│
    └─────────────────────┴──────────────────┴────────────┘

Read each file as a nested vector of lines. A without B, like set operations.