Why is "envy" is considred a sin while "envious" is a rather desirable attribute?

Envy has different connotations, ranging from bad to good. At one extreme, one can be "consumed with envy." At the other extreme, envy can be roughly synonymous with admiration or respect: "Her intelligence and radiant personality made her the envy of her classmates."

Incidentally, I would take that Seven Deadly Sins site with a grain of salt. For example, it lists pride as a deadly sin. While people should certainly strive for humility, is it really wrong to be proud of getting an A on a test? Anger is similarly a mixed bag; I'm proud to hate people who pollute the planet, for example.

There are similarly many great people that I respect, admire and envy, all at the same time.

From Wikipedia, Envy: "Psychologists have recently suggested that there may be two types of envy: malicious envy and benign envy—benign envy being proposed as a type of positive motivational force."

Being "envious" is not considered positive by English speakers. It means to feel envy and as you know envy is not considered a positive emotion. It has a strong sense of resentment at another's good fortune.

Envy is not a good thing for one to feel, it is at best a minor flaw in your character.

But, someone else feeling envy OF you is a bit of praise. This can result in someone stating or admitting to envy as a form of praise “you did such a wonderful job, I’m a bit envious of how smoothly you handled that, I couldn’t do that nearly so well”. This may or may not be literally true, if true, it only reflects well on the speaker as honesty. It reflects well on the person being spoken to, whether the speaker is being totally honest or not, because it implies that one has or did something that is worthy of being envious of.

The worst possibly interpretation of someone is being envious OF you is that they have had it hard and you should probably be a bit sorry for them. For example a childless couple viewing a couple with kids. The worst possible interpretation of being envious of someone else is that you are a terrible person that is not to be trusted because you will act on that envy.

"Positive" and "negative" moral connotations, whether absolute or relative, are not really within the dictionary's area of competency. However, you can expect from it a summary of a word's meaning and history.

Envy means, according to Mirriam-Webster:

1: painful or resentful awareness of an advantage enjoyed by another joined with a desire to possess the same advantage (M-W)

Painful and resentful definitely seem unpleasant to me, making this sort of envy undesireable. This sort of awareness is usually not an advantage, although the "desire to possess" might possibly propel one toward something "good".

The history of "Envy" is equally nasty, and apparently included some moral judgement back in the day:

late 13c., from Old French envie "envy, jealousy, rivalry" (10c.), from Latin invidia "envy, jealousy" (source also of Spanish envidia, Portuguese inveja), from invidus "envious, having hatred or ill-will," from invidere "to envy, hate," earlier "look at (with malice), cast an evil eye upon," from in- "upon" (from PIE root *en "in") + videre "to see" (from PIE root *weid- "to see"). (Etymonline, emphasis added)

Envious simply means

1: feeling or showing envy (M-W, and it always has meant the same)

Jealous, on the other hand, is part of the same etymological family as "zealous" (which simply means roughly "with passion"), but now is often synonymous with "envious":

1: hostile toward a rival or one believed to enjoy an advantage : ENVIOUS His success made his old friends jealous. (M-W)

Again, note that hostile is unpleasant at best, although "jealous" has other uses.

Perhaps the most obvious difference between "jealous" and "envious" is that you can be jealous of your own things, but the object of envy always belongs to someone else.

One might almost say that these two words are used as if they were interchangeable ... The words are scarcely synonymous, however. Envy means discontented longing for someone else’s advantages. Jealousy means unpleasant suspicion, or apprehension of rivalship. —Theodore M. Bernstein, The Careful Writer, 1965 (Quoted in M-W)