Hello, here are step my step instuctions on how to decap someone in single player, Nukem style ;)
Btw, this is my first instructional post, so forgive anything stoopid.

Hold all
Raise both shoulders
Contract both pecs
Contract abs
Contract both hips
Set hands to grab (by clicking them)

Space x2 (you should be holding his head)

Lower both shoulders
Contract both knees

Space x2

Extend both knees
Extend both hips

Space

There, now do whatever you want. The end