The date of the murder was Jan. 17, 2020. It was a Friday, but notably the Friday before a Holiday weekend and the day before the Women's March in DC. My memory is that a lot of people took that Friday off-- either to host visitors from out of town or to get the heck of Dodge and avoid the crowds.
I don't see timestamps on the surveillance video, but the clip of the person of interest walking north down N. Randolph St. towards 9th St. must be of them heading towards the apartment complex. If you're familiar with walking in Ballston, that particular section of the sidewalk is very unpleasant to walk down. It's almost always in shade, or you have intense glare from reflections off the glass windows across the street. There aren't any shops or places to eat on that side of that block. Plus, the balustrades visible on the left side of the frame are to protect the building from trucks entering the huge freight/receiving area. Right next to it is the complex's underground parking garage entrance/exit. You could get stuck waiting a long time for a box truck to back up into the freight area, or honked at by some impatient driver trying to enter the garage. The clips of the person of interest jogging south down the west side of the street on N. Randolph Street towards N. Glebe Road and then crossing N. Randolph street in front of the DARPA building struck me as well. It means that they were crossing away from the main parking garage for Ballston Mall.
This leads me to believe that this person arrived to Ballston by bike. There are a lot of bicyclists that like to lock up where the person of interest seems to be jogging towards. Arlington is pretty notorious for bike thefts, and there's a general belief among cyclists that thieves would be less likely to steal your bike in front of the DARPA building.
If the person of interest is a cyclist, then, for me, that aligns with their attire, their backpack, and especially their choice to wear a balaclava. If you're riding a bike in January in Arlington, VA, you'd want to warm up the air before it hit your lungs. The fit of the poly-material pants is a common look for bicyclists. It would also explain why the person of interest is wearing their backpack so high up (shortened straps). It gives more context for the water bottle in their side pocket too.
(For what it's worth: I worked and biked around in Ballston for over 8 years. I know that area very well.)