Welcome, folks! Now, I really wish we had some Xerneas and Yveltal sprites to add, gah. :D
What do you think about Xerneas and Yveltal, the new Legendary Pokémon?
Love 'em! They are amazingly designed. I see Xerneas potentially being part Grass/Steel/Psychic while Yveltal could be Flying/Dark/Fire or even Poison or Fighting. I believe they could change their types through some move or some in-game event, kind of like Rotom.
Are Legendary Pokémon really capable of being Good/Evil?
Darkrai isn't exactly malicious (at least in the movie, dunno about the games). Those attacks are mostly defense mechanisms. That's Darkrai's power, to induce sleep and cause nightmares. I guess he can disappear or flee, but he may decide to send his opponent into sleep to ensure that he won't be pursued nor bothered.
Legendary Pokemon are sometimes misunderstood, haha. I remember from Lugia's Pokedex entry that it quietly spends its time in the ocean because its wings are strong enough to knock over houses. :( Like LilJz said, the legendary Pokemon often start fighting when provoked rather than engaging in evil activities. Legendaries do what they want and may destroy their surroundings or cause intense droughts or bend time or... whoops!