2R is a good time to learn the Sunk Cost Fallacy: you don’t need to commit to killing an enemy just because you put a bullet or two in one.
Sometimes it’s easier in hindsight, but I think the point of that initial lone zombie in the intro is to show how many bullets it takes to down a zombie for good.
The better option is to stun them. Traditionally a head shot is the way to go, and here there’s a chance of an instant kill which is particularly nice for your ammo count. But there’s a downside, and that’s how erratic the zombie movement can be. An easier target is the legs, and there’s a better stun to be had from aiming there before taking the opportunity to run past.
Either way, the 1-3 shots for a stun is much more cost effective than ~15 to kill, and the risk taken by downing but not killing is a big one. When one of those surprises you by climbing up your leg and chews your face, you’re going to wish you just stunned it and ran past.
Or that you had a shotgun. Boom, headshot.