How is phenol antiseptic?
The main reason for phenol being antiseptic is it's effect on proteins which it denaturates. It also causes leakage of cellular membranes. See the paper in reference 1 for some details and a load of references. They say in the section on phenol:


  Phenolic-type antimicrobial agents have long been used for their
  antiseptic, disinfectant, or preservative properties, depending on the
  compound. It has been known for many years (215) that, although they
  have often been referred to as “general protoplasmic poisons,” they
  have membrane-active properties which also contribute to their overall
  activity (120) (Table ​(Table22).
  
  Phenol induces progressive leakage of intracellular constituents,
  including the release of K+, the first index of membrane damage (273),
  and of radioactivity from 14C-labeled E. coli (242, 265). Pulvertaft
  and Lumb (386) demonstrated that low concentrations of phenols
  (0.032%, 320 μg/ml) and other (nonphenolic) agents lysed rapidly
  growing cultures of E. coli, staphylococci, and streptococci and
  concluded that autolytic enzymes were not involved.


Reference:


Antiseptics and Disinfectants: Activity, Action, and Resistance