Question: I'm writing about a man that learns he is a government test subject then becomes a monster because of it and that change makes him insane how do I make him seem insane to the reader?Answer:
There are many types of what is commonly called insanity, and if you want to portray a particular type of mental illness, you should probably do some research on the type you're interested in so that you can describe the character's thought processes and behaviour accurately.
On the other hand, what is commonly presented as insanity in fiction is a mental state in which a character may misperceive or lose touch with reality (perhaps live in a fantasy world), draw and act upon illogical and false conclusions which no sane person would entertain, have emotional responses (such as anger) that are exaggerated and drastically at odds with the situation, or lack empathy to the point that he/she can behave cruelly to others with no remorse or guilt.
As I say, this is how insanity is often portrayed in fiction. It often has more to do with behaviour that the reader or the other characters find frightening or that makes them see the insane character as untrustworthy than with an actual mental illness. But if you simply want to make a good villain, that may be all you need.