When you remove an Axolotl, on which human(s) do you call removeAxolotl()? – DMGregory♦

Only the human who lands the killing shot. Do you have any suggestions for making it run on all of them? – Crystalline787

That's a problem, because this method contains a very important piece of code:

closestAxolotl = null;
lowestDistance = 0;

All your other humans who were attacking that axolotl don't get their lowestDistance variable reset to zero.

Since you only reset this variable to a higher number when its value is exactly zero, and you only change targets to ones whose distance is lower than the current lowestDistance, if a human was close to an axolotl that died without being the one to land the killing blow, it will refuse to switch targets to any axolotl farther away than that now-corpse.

You can make this code much more robust (and more efficient) by not storing lowestDistance between frames. Less state to reset!

// Don't put this check inside your for loop:
// code inside the loop only executes if Count is > 0
if (axolotls.Count == 0) {
    myAgent.SetDestination(randomDestination(randomWander));
} else {

    // Initialize our search to the worst possibility: 
    // no target, infinitely far away! (Literally ANYTHING else is better!)
    float lowestScore = float.positiveInfinity;
    Axolotl closest = null;

    Vector3 myPosition = transform.position;
    foreach (var axolotl in axolotls) {
        // Skip destroyed axolotls (better: make sure they're not in the list!)
        if (axolotl == null) continue;

        // Don't pay for a square root we don't need.
        float score = (axolotl.transform.position - myPosition).sqrMagnitude;

        if (score < lowestScore) {
            closest = axolotl;
            lowestScore = score;
        }
    }

    // "closest" now contains the nearest axolotl.
}

There are some other things in your code that don't make sense, like this line, which removes an axolotl from a list, only if it wasn't in that list:

if (!axolotls.Contains(closestAxolotl))
{
    axolotls.Remove(closestAxolotl);
    // ...

...so I think you should take a thorough proofreading pass through the whole file, or maybe get a friend to do some code review with you, to make sure you don't have more bugs lurking in there.