Most viruses do not kill their host, in fact some will actually go into a lysogenic cycle in which they lie dormant in the cell until a set number of factors come along and the viral genes begin to be expressed. HIV is a great example of this.
Viruses need to be passed on from Host to Host, which answers part of your question, "Why do they kill their host?". Cells are bound by tough membranes which require some form of energy input to escape, since the average bacteria is bigger than the smallest pore on the cell surface they must lyse the cell membrane to escape to infect other cells.
However, sometimes the bacteria will rewire the hosts chromosome giving it an ability it normally didn't have before.
An example of this would be E.Coli 0157:H7. It is believed that a Shigella phage somehow transfected an E.Coli cell and caused it to begin to express Shiga-like toxins.
Viruses need us, like we need Viruses. They help control bacterial, fungal, and prokaryotic populations.
Your question is wrong. A virus does not usually kill its host.
A virus does not care if it kills the host or not as long as it lives long enough to infect the next host. Either way, the old host will be no use because it will either be dead or will have gained immunity. A virus just needs a supply of new hosts.
not all virus kill there host well those who do will just find another one or die after reproducing
Not all viruses kill their host. Often deadly viruses will evolve to be less virulent (less deadly) so the host stays alive to spread more virus.
Here's an example about West Nile virus: http://www.accessexcellence.org/WN/SU/wnvattenstyjan05.htm
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