The missive establishes, if there were any doubt, that Trump hates impeachment with a passion, and he expresses his contempt in his own inimitable voice — scornful, hyperbolic, colloquial, on brand (Russia Witch Hunt, Do Nothing Democrats, etc.) and, above all, aggrieved.
We've never had a president in the modern era who has actively cultivated an image of victimhood, a posture that once would have been considered whiny and weak, but that Trump has, through his personal alchemy, made into a kind of political strength.
It is inevitable that a president — carrying out impossibly weighty responsibilities, in a very public capacity, while the opposition and press fasten on every peccadillo — is going to feel besieged, misunderstood or otherwise unfairly treated.