Okay, so, this is patently untrue. First off, I don't know what your definition of a virus is but it's way different from what everyone else agrees a virus is. Second, android itself CONTAINS task killing facilities in the applications area of the settings menu, it's just so clunky and hard to get to that a task killer app is useful. Three, run and killing tasks are a basic OS function - if android can't handle a request to close an application, it is a POS operating system.
It's definitely perfectly fine to kill tasks on your own. Android does a certain amount of garbage collection and memory maintenance on its own, but it's not perfect, and a runaway task eating up memory and CPU could easily look like an important and active process and never get swept away by the GC.