Abilities and personality should mostly come from Pokedex entries I think.
Pokédex entries can only explain so much. For example, while it's said that Giratina was banished for its violence, it's not known whether or not it still
is violent or what it did to be deemed violent in the first place. (It could have, for example, accidentally screwed with reality just by sitting still. Some fans
do think it's the legendary of anti-matter, after all.) Moreover, how it acts in the games isn't necessarily how it acts in other canon. Example? In the movies, while Giratina was rather ticked off, it was understandably so because the other legendaries were messing around with its home. Once it settled down, it
helped straighten things out in the real world by defending everyone else from Arceus (who was pretty much batcrap crazy and trying to kill everything that moved at the time).
Then, there's also pure fan interpretation from that. A lot of fans argue that there may be more than one legendary. (Example: The anime shows more than one Lugia, and not all of them have the ability to churn hurricanes with a flutter of their wings like the 'dex says they can.) For that reason, one can argue that each legendary has their own unique personalities. That's why it's easy to buy one fan's interpretation of a legendary, even if it's not quite consistent with what we're used to. For example, if Gothitelle wanted to write a fic about a bubbly Giratina, she
could say that it's just not the same one as the one seen in the movies or the one described in the 'dex, and that would be okay because it's a lot like saying one human isn't the same as another, if that makes sense.
I guess what I'm trying to say is that while there's certain bits from the 'dex that you just can't change (like how Growlithe might get territorial or how Clefairy are attracted to moonlight), but other bits (like bits about personalities) are a bit harder to pin down as the final word concerning that Pokémon. The best way to go about writing any character within this universe is by sitting down, going through everything you can concerning the cast you want to use, and figuring out from there what your take is and how you can get everything to make sense. Don't just stop at the 'dex if you want to be canon-accurate. Check out the anime, manga (if you can find it), games (some clips of which are on YouTube), anything you can find.
Meanwhile, I do agree that there needs to be a good reason for making a character a certain way, but this is generally true unless you're writing complete crack. It's just easier for a reader to grasp a certain personality if it makes sense in context. While Distortion World isn't necessarily Hell, it
does have certain qualities to it that Gothitelle should keep in mind while developing Giratina's personality (because those details can lend a hand towards how Giratina sees reality). I'm being vague at this point because even the Distortion World is interpreted differently across different canon, and not every fan thinks of the same thing when someone says "Distortion World." If you really want to see someone having fun with the concept of an off-beat Giratina, I'd suggest checking out Wings Have We and other fics
by Giratina. She uses a Giratina that isn't as violent, broody, or psychotic as the 'dex would imply.
Long story short: not every person is the same, not every canon is the same, and not every fan interprets canon the same way. 'Dex entries shouldn't be the only thing you stop at when it comes to developing a Pokémon character.