How should I write?
The house was ignited.
The house was burned.
The house was kindled.
The house was inflamed.
The house was lit by fire.

I want to say that the house burned but not accidentally, someone put fire on the house on purpose.

Question

How should I write?
The house was ignited.
The house was burned.
The house was kindled.
The house was inflamed.
The house was lit by fire.

I want to say that the house burned but not accidentally, someone put fire on the house on purpose.

Answer

Someone set the house on fire.
The house was set on fire.

Perhaps a little more informal.... Someone torched the house.

Answer

The last one most closely means that it was intentional. But Jon is right. By saying "someone lit the house on fire" or "someone burned the house" is a better way to say that. 

P.S. The word "arson" is the act of intentionally setting something on fire.

Answer

Can't you just say "someone"? None of those imply whether or not it was accidental or intentional.

"Someone burned down the house" - This, at least implies, that is was intentional.

Answer

There was arson involved in the house fire

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