Why did Descartes called his thought experiment "evil demon"? What if we lived in a simulation that turned out to be more pleasant than reality itself (eg. The matrix series) and it would be better from an emotional/psychological point of view to remain ignorant? Why did not he call it maya, world of delusion or similar? Was he forced by his cultural environment or perhaps he thought that lying or keeping people ignorant of their true nature is always unethical?
The original Latin phrase Descartes used in the Meditations is genium malignum. In the later French translation, approved by Descartes, this was rendered as mauvais genie or as nearly everybody now says, malin génie. This is our much-vaunted 'evil demon'.
The evil of the evil demon
A proper first step is to realise that Descartes's major purpose in the Meditations is to arrive at beliefs which are immune from error. Perhaps there are no such beliefs, I might add, but Descartes wants to reach them if they are to be had. Anything that deflects him from the pursuit of beliefs which are invulnerable to error undermines or in fact ruins his purpose.
Descartes imagines in a thought-experiment in Med.I that there is 'some malicious demon of the utmost power and cunning [who] has employed all his energies in order to deceive me'. The demon can so contrive and manipulate Descartes' experience that nothing is as it appears to Descartes to be. Either whatever Descartes believes to exist doesn't exist or what does exist has has none of the attributes Descartes believes it to have. The demon puts Descartes under a kind of universal deception.
The demon is evil relative to Descartes' purpose or project of arriving at the truth about the matters encompassed in his inquiries. The demon is hostile to, and makes futile, the pursuit of beliefs which are immune from error. If the demon has this power and exercises it, he subverts the Cartesian epistemological enterprise.
In the event, disclosed in Meditation II, the demon's 'utmost power' is limited; he cannot make Descartes believe that he (Descartes) exists when in fact he doesn't. If the demon instils this belief it is necessarily false and recognisable by Descartes as such, since (per impossibile) Descartes has to exist in order to acquire the belief that he doesn't exist.
The demon elicits the conditions for maximal happiness
This is not a Cartesian idea but (along the lines of your jeux d'esprit) if we remember that the demon's purpose in the Meditations is only to deflect the believer from truth and that this is what Descartes has against the demon, the demon could use his power and cunning to create a world in which false belief prevails but also in which this state of false belief creates supreme happiness. This is logically possible. The Cogito does not work against this since the demon can presumably cause us never to think of the Cogito and of the beliefs Descartes elaborates around it.
Solipsism is irrefutable. Descartes most probably knew that his argument does not reach very far, so he had to rely on qualifications: a good non-deceiving deity and an evil one. "Unum verum bonum" was the ideological slogan which in the European early 17th.c has become really unconvincing. Nobody upheld the negation (plural, false, evil), but scepticism has become a widespread attitude.
(Btw currently wikipedia says: "Many people are intuitively unconvinced of the nonexistence of the external world from the basic arguments of solipsism, but a solid proof of its existence is not available at present.")