@Kanol
Well you think about it, if u cant kill the zombies which you seem like you can't despite the many headshots. killing the humans will be effective since there will be one unkillable zombie instead of five unkillable zombies in case you forgot it was because the big guy got infected that they were able to break down the cell door.
When you kill someone depending on how the muscles react it sometimes won't let go, muscles don't immediately just go limp sometimes they can stiffen resulting you being trapped even if you blew that guys head off.
Exterminating rats takes a long time and its a repetitive process. from what I gather from the story you aren't allow to be leaving the premises to salvage stuff so they probably dont take the same route or exit everytime, the way those guys reacted to finding that big metal piece made it look like it was their first time discovering it.