Dead Island: Riptide Definitive Edition

This is from the fanmade "Dead Island Wiki" so I won't take credit for it. I'll put a link to the page at the end of this solution.

"The Chainsaw returns in Dead Island: Riptide. It is available from the first time you visit Mataki Village on a quest for Marcus Davis early in the game. After accepting a quest for Dave called "Genre: Horror Slasher", you can find the chainsaw in the abandoned movie set. A second chainsaw, a level 25 orange chainsaw, can be found by defeating the final boss in the game [Harlow]. However, while the orange chainsaw is set at level 25 permanently, the regular chainsaw spawn in the movie set will scale with your level, meaning you can return when you are a higher level to collect an upgrade thus making the end game chainsaw obsolete."

This sums it up pretty well on how to get a chainsaw. Now, once you have it, go ahead and kill 25 zombies with it - it does not matter if it's a Walker or an Infected.

Link to the fanmade wiki-page: https://deadisland.fandom.com/wiki/Chainsaw