Outsider (Lawful) or Outsider (Good, Lawful)?
I'd say just Lawful, going by the description.
Their reasoning "lawless creatures would be better dead" doesn't suggest they're that Good.
That's reasonable.
We can make the alignment "Always lawful, often good" to account for the original good tendencies.
Before we get into special abilities, let's figure out the basics so I can get a Homebrews started.
Int is given: 16
Other mental scores are probably within a few points of that.
Dex should be high. It has fantastic AC, and is blindingly fast with its whirlwind attack. Maybe Dex 20 or even 22?
Str seems like it might be slightly above average, based on its 1d10 damage.
I see no strong indicators for Con.
I'm not sure the shape changing quite fits alternate form or change shape. And it doesn't have much mechanical benefit. Maybe make it "change form" and maybe allow a minor benefit (some forms have a Climb speed, etc)?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.