Many "free" mods replace original ads with even more intrusive ones that run in the background, ironically draining the battery the app is meant to save.

Modded apps do not receive official security patches from the Google Play Store, leaving your device vulnerable to newly discovered exploits. Safety Recommendations

Here is a report on the risks and functionality associated with such files:

KillApps is a legitimate utility for Android designed to close background tasks, free up RAM, and save battery. The "Pro" version is a paid upgrade that unlocks features like automated closing, system app management, and an ad-free experience.