Hi, don't know if you're still bothered by the problem but as I experienced the same issue the following works for me anyway. Disable your SIP (System Integrity Protection) by booting into Recovery Mode, open Terminal, punch in "csrutil disable" and hit enter. Restart your computer, and remove that "avast.app" with Terminal command "sudo rm -rf [insert file path]". Voila. After it's done if you feel the need to re-enable SIP (which I did anyway) just again, boot into Recovery mode and use "csrutil enable" in Terminal; then restart. Everything should be fine.