Since KillApps requires Accessibility Services permissions to function, a compromised version could potentially "read" everything on your screen, including passwords and banking details.
These files are often injected with malicious code. According to security researchers at Malwarebytes, modified apps can include trojans that steal personal data or monitor your keystrokes. killapps-pro-close-all-apps-running-mod-apk
If you have already downloaded an APK, you can upload it to VirusTotal to check it against dozens of antivirus engines for hidden threats. killapps-pro-close-all-apps-running-mod-apk