Why kill Max?

Higher body count for the horror fans.

When I first saw this movie I thought of the same thing! After rewatching it a few more times......I still didn't know ha ha ha! But then, almost 20 years later I finally got my answer per the trivia section for this movie ?

From the trivia section:
The last thing that Max says to Barry is that he is going to turn him in to the police. It is later established that the killer had followed the four leads to the fishing pier and had taken pictures. This implies that he had witnessed the scene where Barry threatens Max and finally learned that Max was present when the car accident happened. This provides a motive about the murder of Max, since the police would have taken Barry under custody for the assault against Max or at least would have watched him. Further investigation about the involvement of the five teens in the fatal night with possible links to the David Egan drowning case would have led to even more interrogations and undoubtedly the killer's plans would have been foiled since he had only a couple of days before the 4th of July. This explains why Max was dispatched right afterwards while the killer continued to stalk the other 4 leads knowing that they could not resort to the police.

It was all about how they thought Max was the one coming after them and when he got killed they knew he wasn't the one. It was an integral part of the plot.

The producers wanted Max to be killed off so there would be a death scene earlier in the movie. In-universe the explanation could be that the fisherman overheard the argument between Max and Barry and feared he knew too much.

+ body count

The main reason was definitely because Max said he was going to the cops, if the cops were watching Barry then Willis would not have been able to carry out his revenge. It may have also been so he could draw suspicion towards Ray being the killer and if the other 3 suspected Ray it would drive a wedge between them making it easier to kill them

One reason only - to give us an extra kill.. had they not killed Max then this movie would have been too light on murder

IKR???
It's so stupid when they do that, they kill people with no connection to what happened... when they're not even witnesses to something, they're just there to die.
It's like the movie Urban Legend, despite being fun, many characters die for no reason.
This is not some "crazy" serial killer... it's just a normal man looking for revenge