There are essentially three reasons why it is argued that D&D is evil.
1. It causes the participant to disconnect from reality into a fantasy world.
This is also the case with good fiction, video games, tv, and even chess. This is by no means a problem all the time. For D&D and any other means off entertainment may be a harmless relaxatin, and may even excercise usefull skills, like the ability to use logic, that apply both to the game world and the real world. This only becomes a problem when the player centers his life around D&D/TV/chess/etc. This is the problem, not D&D.
2. The players have their charachters engage in evil behavior such as wholesale slaughter, torture, pacts with deamons, and in the more extreme arguments, any casting of spells. This will eventually lead to undertaking these activities in life.
As for the casting of spells there is a simple counterargument. The properties of the fantasy world are different than the real world. The magic of a fantasy world does not corespond to that that adherants claim exists in the real world. So why must fantasy magic require the trafficing with deamons that real magic supposedly does. Similarly, the violence of combat may be neccessary in a fantasy world with different social structures and types of enemies without it being any more acceptable or desirable in real life.
As for evil acts, there are several ways to escape this problem. One is to say that simply you do not play an evil charachter. Although evil as rules, players are expected to be good, noble and heroic. Another is to say that D&D is a way to vent steam and have a kind of cathartic experiance without causeing real world damage. Or one could still study evil in oneself and man in order to understand it, so that one may be combat evil in oneself and others in the real world. Thsi is a similar approach as the christian study of demonology.
The evil in D&D is only a problem when it is held up as noble and heroic, or when it is taken up by people with severs psycological disorders that cannot distinguish reality and fantasy.
3. D&D players kill themselves and commit other crimes much more often that the general population.
Given suicide and murder rates at several per 10,000 pop per year, and no more than the same 10 cases cited in any article, compared to the 10s of thousands of D&D core books that have been in circulation for over 10 years. Even assuming only one player per PHB, the crime rates seem to be LOWER than average.
Even if this were not the case, it could be argued that D&D attracts the disturbed, not creates them. It would then be as valid to claim that prison creates criminals because of the high concentration therein, as D&D creates insanity.