I think I would keep that simply to someone who burns a house down because they wished to cause harm (physical or mental) to the occupants is more serious than someone who burns a house down because they like playing with fire but had no harmful intent against its owners/occupiers, and not bring the reason why into it at all (other than possibly premeditation/revenge issues).
Whilst I agree that anybody who burns a house down hoping those inside are killed or injured is a serious offender and needs help . I still think there are grounds to differentiate between for example .
1) Someone who burns down a house with their ex wife and new partner inside because of jealousy
2) Someone who burns down a house because it is full of someone of a particular race/nationality and the arsonist harbors deep hatred for that race/nationality .
The person in 1 needs treatment for anger management issues . The person in 2 needs treatment for serious xenophobia. The person in 1 is probably also only a risk to a smaller number of people in their own direct circle . Wheras 2 is a risk to a large population of people in the same class as those they hate .