You would only create these viruses if the total utility of the viruses you can create with the resources at your disposal exceeds the utility of the humans you could make with these same resources.

True, if you start with resource constraints, you can rig the utility scaling to overweigh more intelligent life. However, if you don't cheat and assign the weights before considering constraints, there is a large chance that the balance will tip the other way. Or if there is no obvious competition for resources. If you value creating at least mildly happy life, you ought to consider working on, say, silicon-based life, which does not compete with carbon-based life. Or maybe on using all this stored carbon in the ocean to create more plankton. In other words, it is easy to find a case where preassigned utilities lead to a runaway simple life creation imperative.