Everyone has an opinion. lol. IMO, I lean toward killer being male disguised as a female. The walk up to the driveway seemed to be with purpose. The killer cowardly fired four shots into Liz and ran from the murder scene, and that sprinting appeared to be male.
Respecting your opinion as it is 110% percent valid. I also agree, the stride when running away seems masculine.
But..... I am going for Occam's razor: Apparent feminine dress means a female perpetrator.
Now, for the masculine appearing stride....
Girls who participate in sports are taught from an early age by coaches to lose their natural feminine running stride when playing sports and to adopt the more efficient masculine stride.
Going out on a
loudly creaking limb (just been around lot of junior high, high school and some collegiate athletes)......
The perpetrator appears not to be heavy set, but athletically "big boned" as evident by the fast stride when fleeing. Furthermore, the stride seems slightly mincing- not the longer stride of a sprinter or distance runner.
Some sports use mincing steps to often change direction fast. Softball in particular features slightly mincing steps when base running- no time to break into longer strides, need to reverse and get back to previous base, react to erratically bouncing balls etc. Softball also has a uhmm.... "certain number" of athletically big boned girls.
So.... though the limb may have broken:
I am thinking 25-30 year old female perpetrator who played softball in high school. She might have also played at the junior college or Division III college level. Or, perhaps plays in an adult recreational league.