In the phasing shell images, Nikita is shown killing her step father by holding him down and suffocating him. However, in the close-up, Peta's hand is only covering his mouth, while leaving his nose to breathe just fine. This wouldn't kill him, at most it would simply shut him up but---because he is able to breathe through his nose---the man would have had enough oxygen to get up and fight back against Nikita.
Walter objects to anyone being used as guinea pig to test the phasing shell. The very next scene, taking place just a few minutes later, Nikita enters the room as the obvious test subject. Walter offers no further protest. This is unlike Walter: he is very protective of Nikita and he had just expressed concern over the effects of the machine. It is unlike him to allow Nikita to go in there, without so much as telling her to be careful or objecting more vehemently to Madeleine or Birkoff, the latter being in the test room with him.