Well, duh. Unix doesn't let you send signals to processes you don't own. Would you want other people to be able to kill your running processes?

You could use 'su', but then you need to supply a password, which you cannot supply from the command line. Consider installing 'sudo', which you could configure such that you don't need to give a password to be able to issue the kill command as root (or as another users). But that would kind of defeat the purpose of having different users, doesn't it? It's certainly a security concern.

Abigail