Since you run acpid as root, it can't start it on user tc's display - X security.
su tc -c "exittc"
Would run it as user tc, which should work.
Interesting.
For those that wonder what the "-c" option does:
"Among the most commonly used of su's few options is -c, which tells su to execute the command that directly follows it on the same line. Such command is executed as the new user, and then the terminal window or console from which su was run immediately returns to the account of the former user after the command has completed execution or after any program that it has launched has been closed.
If the command inclusive of its options has one or more spaces in it, then it must be enclosed in quotation marks; otherwise, su will interpret whatever comes after the first space in the command as the name of a user to whose account su is supposed to switch (the result of which will be an error message). "