You’re right that viruses don’t “do” anything on their own, they get our cells to do things for them. But calling a virus a “chemical” (which usually implies a substance composed of a single type of molecule) is also overly simplistic. The genome of viruses can be quite sophisticated and can instruct our cells to do lots of things that are to the virus’s benefit, like making certain chemical messages that interfere with the functioning of our immune systems, helping the virus to evade detection.

A prion is closer to what you’re describing, consisting of truly just a single protein.

The line between “alive” and “not alive” is somewhat fuzzy and viruses straddle that line even though we classify them generally as “not alive”. They don’t have their own energy metabolism but they do have their own genetic material, and undergo evolution and “survival of the fittest” in the same way as life does. So the virus has evolved to “make” its hosts aggressive because it helps it spread. In a sense you can say that the virus does have a “reason” to do so so that it can reproduce.